Could Ronald Bolanos be a sleeper for the Royals in 2021?

When the Royals traded Tim Hill to the Padres last summer, Franchy Cordero and his prodigious power potential seemed to resonate most with Royals fans. However, often overlooked in the deal was right-handed pitcher Ronald Bolanos, a highly-heralded arm in the Padres system who was signed for $2 million out of Cuba during the 2016-2017 international signing period. Even though Bolanos was considered a surplus arm in the Padres’ deep system, he rates pretty decently among the Royals’ own prospect lists, as MLB Pipeline ranks him as the Royals’ 17th best prospect. Considering pitching is a source of strength for the Royals system, the addition of an arm like Bolanos should give Royals fans hope that there is plenty of depth pitching-wise in the organization in both in the near and distant future.

While Cordero seemed to impress in limited action in 2020 (he missed most of the year due to a wrist injury), Bolanos struggled in two appearances and 3.2 innings of work in his Royals debut. Pretty much every metric of Bolanos’ 2020 looks pretty ugly at first glance: 12.27 ERA, 12.46 FIP, 0.67 K/BB ratio, and 50 percent HR/FB rate. Of course, with only 3.2 innings of work, it is a small sample, and it’s hard to take those metrics too seriously. That being said, Bolanos did pitch 19.2 innings in 2019 with the Padres, which is a much healthier sample than last year with the Royals. Unfortunately, the metrics from that stint still aren’t all that great either: 5.95 ERA, 5.25 FIP, 1.58 K/BB ratio, and 18.8 HR/FB rate.

Hence with 23.1 IP under his belt, the early results for Bolanos haven’t been impressive. However, Bolanos is only 24-years-old, and he has gone through some development in his mechanics in just one short year with the Royals. Thus, while many Royals fans may not be thinking about Bolanos as someone who can contribute to the Royals pitching staff in 2021, it is possible that he could be a dependable arm who could emerge mid-season, especially if injury or ineffectiveness besets anyone in the Royals rotation or bullpen.

Let’s take a look at Bolanos’ profile and his progression as a pitcher from his days as a Padres in 2019 to his brief stint with the Royals in 2020, and why that progression could be a sign for a possible improvement on the mound in 2021.


Bolanos has always possessed a live arm and strong potential, especially in the eyes of scouts. Here is what MLB Pipeline said about Bolanos in their most recent scouting report:

At 6-foot-3, Bolaños features a strong starter’s build, and he could still grow into his frame a bit. His fastball sits around 94 mph, though it can touch 97. Bolaños has made strides with his secondary options as well, particularly his breaking pitches. Both are high-spin offerings, with the slider sitting in the mid-to-high 80s and biting sharply, while the curve drops but still hits 80 mph. During his brief time in the big leagues, he used both to some success.

“No. 17: Ronald Bolanos; Royals Top-30 Prospects”; MLB Pipeline

In 2019, Bolanos’ curve ball ranked in the 72nd percentile in spin rate, which was pretty impressive. In fact, his curve ball overall was one of his best pitches in 2019, according to Baseball Savant. Bolanos threw his curve ball 20.5 percent of the time with the Padres, and the pitch had a wOBA of .236, a xwOBA of .191, and a whiff rate of 55.2 percent, all impressive metrics. Furthermore, as evidenced from his heatmap from 2019, Bolanos also showed excellent command with the pitch in the lower part of the strike zone.

What made the pitch so effective in 2019 was the big difference in velocity from his four-seam fastball. In 2019, Bolanos’ FF averaged 94.3 MPH, which ranked in the 70th percentile. The curve? It averaged 76.4 MPH, nearly an 18 MPH difference. In the GIF below, it is obvious that Cubs slugger Anthony Rizzo didn’t pick it up properly, and thus whiffed badly, as a result:

While the curve ball proved to be an important pitch in Bolanos’ arsenal, his FF didn’t prove to be nearly as effective in 2019. With the Padres, Bolanos’ fastball produced a wOBA of .372, a xwOBA of .431 and a whiff rate of only 12.5 percent. Bolanos pretty much located the pitch up in the strike zone as well as in the middle, both places where hitters can feast, especially if they are looking for it. Here’s a glimpse at Bolanos’ fastball heatmap from 2019:

Let’s also take a look at the pitch in action in a GIF below. He does lay it up in the zone against Dodgers hitter Will Smith, which is not a promising sign, initially. However, the pitch does have some tailing inside action, and that causes Smith to pull it on the ground for an easy ground out rather than a base hit.

Speaking of tailing action, it should not be surprising to know that Bolanos also sports a sinker in his pitch arsenal, which he threw 8.1 percent of the time in 2019. In fact his sinker was his second-most effective pitch in 2019, as the pitch sported a wOBA of .272 and whiff rate of 25 percent. Granted, the pitch did have a xwOBA of .354, which is nearly 82 points higher than his actual wOBA. This difference shows that Bolanos may have gotten a bit lucky on the pitch two seasons ago in San Diego. Nonetheless, Bolanos’ sinker and curve ball at least hint that Bolanos has the potential to sport two plus-pitches that can get batters out at the Major League level.


One of the biggest differences for Bolanos in his transition from San Diego to Kansas City was his decision to forgo the windup and pitch solely out of the stretch. Bolanos is not the lone Royal to do this, as Duffy did this a couple of years ago as well, and Jakob Junis also modified his windup to incorporate a sidestep rather than a step behind the mound to begin his windup, which he did previously. The Royals and their coaching staff have always encouraged pitchers, especially new ones to the organization, to tweak things here and there, and Bolanos seems to be no exception.

As evidence, here’s an example of Bolanos pitching out of the stretch with nobody on and striking out White Sox second baseman Nick Madrigal:

A couple of things stand out about Bolanos’ adjustment in 2020. First off, he is averaging nearly 97 MPH on the fastball here, and it looks like he is throwing it with more ease in comparison to his Padres days. Unlike his windup, where a lot seems to be moving around, his pitching motion out of the stretch is a lot more refined, which should bode well for more long-term success if he continues to keep this approach. Furthermore, the switch also had an effect on his velocity overall, as his FF averaged 95.2 MPH in 2020, a 0.9 MPH increase from 2019.

Another big development for Bolanos was the increased usage of his slider, as he threw it 28.8 percent of the time in 2020 in comparison to 11 percent of the time in 2019. In fact, his slider and curve ball usage flipped in 2020, as his curve ball usage dropped from 20.5 to 9.6 percent from 2019 to 2020. Considering his curve ball was his most effective pitch in 2019, it was surprising that Bolanos utilized it less. However, there is some strong break on his slider, and when he locates it well, it can be tough on opposing hitters, as evidenced in the GIF below when he went up against eventual 2020 MVP, Jose Abreu:

Another development for Bolanos as a Royal was the increased usage of his sinker, as the rate went from 8.1 percent in 2019 to 15.1 percent in 2020. That being said, Bolanos didn’t experience as much success on the pitch in 2020 as he did in 2019 with the Padres. After posting a wOBA of .272 on the pitch in 2019, the wOBA on the sinker inflated to .450 in 2020. On the other hand, the sinker did have a xwOBA of .423, which shows that he may have been a little “unlucky” on the pitch, which was the opposite situation in 2019. Hence, the effectiveness of sinker may fluctuate year to year and may hinge on what wOBA-xwOBA difference for that that particular season (especially since it doesn’t seem to be a dramatic whiff-inducing pitch).

That being said, when he did locate the sinker properly, Bolanos was able to get out of jams, as evidenced by him inducing Cleveland’s Yu Chang into an inning-ending double play:


As of now Bolanos throws five pitches (FF, SL, CU, SI, and CH), but he may be on his way to being a four-pitch pitcher in 2021, as he only threw his changeup 1.4 percent of the time, which was down from 5.2 percent in 2019. In fact, it could be possible that Bolanos may refine his pitch arsenal even more, as he may need to make a decision in regard to the sinker. If it truly is a pitch he trusts, he may want to phase out his FF a bit and use his sinker more, in the mold of Brady Singer in 2020. On the other hand, he could reduce his sinker usage and focus on developing more velocity and command behind his FF in 2021, which may be more beneficial, especially if he can average 96 MPH or above on the FF consistently.

What Bolanos does with his pitch arsenal could depend on what track the Royals have for him in 2021 and whether they think he still can be a starting pitcher, or if he would be better utilized out of the bullpen. Right now, it’s hard to say what Bolanos’ better path may be as of this moment. Obviously, he has the stuff to be a decent middle innings reliever down the road at the very least, and that may be the quicker route to the Majors. However, he does have the ability to throw three-to-four pitches effectively, which shouldn’t be undervalued. Most relievers thrive as two-pitch pitchers, with a third semi to decent one to mix things up in the late innings. Bolanos’ current five-pitch mix is quite diverse, though it seems like he hasn’t really found a “go-to” pitch just yet, which make the Royals a little apprehensive in terms of moving him to the Major League bullpen too quickly.

As of now, it’s likely that the Royals will start Bolanos in Triple-A Omaha, barring a spectacular Spring Training campaign. That being said, I think it would benefit him to continue to develop as a starter, and see what “go-to” pitch (or pitches) emerges in his work as a starter. I do think that 2020 is an unfair sample to judge him too harshly on, as many Royals fans have done. Furthermore, his 2019 sample wasn’t a whole lot either, and as Royals fans can see in the GIFs above, his pitching motion and mechanics are different, which shows that Bolanos is far from a finished product on the mound. Bolanos’ Royals debut wasn’t pretty, but he still has plenty of room to develop, and some more innings in Omaha may help him become a valuable weapon for the Royals as soon as next season.

In fact, it’s not difficult to imagine Bolanos perhaps emerging as a swing man mid-year or after the All-Star break, especially if he gets some much-needed development and confidence with the Storm Chasers after a lost Minor League season in 2020.

And for the Royals in August to have a 25-year-old arm who can fill in for a spot start or eat innings in the middle of a game if the starter gets knocked out early?

Well…let’s just say the Royals wouldn’t shy away from cherishing an asset like that on their pitching staff both in the short AND long term.

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