Projecting Pittsburgh Pirates Pitcher Mitch Keller’s Ceiling and Floor


Pittsburgh Pirates right-handed pitcher Mitch Keller is very talented and has the promise of an ace. What could the ceiling and floor of his performance be?

Currently, the Pittsburgh Pirates best pitching prospect is right hander Mitch Keller. He is currently a consensus top 50 prospect across the MLB, and has previously touched the top 20 in terms of rankings. It’s very arguable that Keller should still be in the top 20-30 top prospects.

Keller was an ace with the Triple-A Indianapolis Indians last season. He posted a 3.56 ERA, 3.60 FIP, and 1.24 WHIP across 103.2 innings. He struck out 123 batters, and walked just 35, which resulted in a strong 3.51 K/BB ratio. Even though nearly 40% of all batters with at least 300 plate appearances at Triple-A having an isolated slugging percentage of .200+, Keller let up only nine home runs as only 9.6% of all the hits he gave up left the park.

Keller ranked as a top-3 pitcher at Triple-A. Among pitchers with at least 100 innings pitched, the 23-year-old was 5th in ERA, 1st in FIP and xFIP, 5th in WHIP, 1st in strikeout rate and K/9, 8th in K/BB ratio, 2nd in HR/9, and 4th in opponent batting average. He carried over most of his peripherals when he made his Major League debut, which proved to be extremely unlucky.

Yes, Keller had a 7.13 ERA, but ERA can be a very flawed stat. It doesn’t take into account the bad defense that was behind Keller in most of his starts (Pirates ranked last in UZR/150, UZR, range runs above average, and 24th in  defensive runs saved), nor does it take into account quality of contact or overall skill of the pitcher such as preventing free passes and keeping the ball out of play, and not giving up hard hit contact.

In his 48 innings of work, Keller struck out 65 batters, while walking only 16. Keller’s 4.06 K/BB ratio would have ranked in the top 25 of all starters in the MLB had he kept it up over a whole season. He also gave up home runs at a 1.1 per 9 rate. That’s actually below league average for 2019 standards. The league average HR/9 was 1.4, which has been on the rise for the past handful of seasons. In 2015, the average sat at just 1.0. Again, Keller would have ranked in the top 25 of all pitchers if he had played a full year.

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All projective stats also said Keller was better than his results suggested. Keller posted a 3.19 FIP or fielding independent pitching, and 3.78 SIERA or skill interactive ERA. In comparison, Hyun-Jin Ryu posted very similar numbers to Keller with a 3.10 FIP and 3.77 SIERA, and he led the league in ERA. Baseball Prospectus also posted his DRA, or deserved run average at 4.18. If you look at his batting average on balls in play, you can see why most ERA predictors are putting him in the 3-ish range. Keller had an opponent .475 batting average on balls in play, or BAbip. Yes, that was not a typo. A pitcher has posted a BAbip of .350+ only 38 times in the history of baseball with at least 150 innings pitched, and 11 since 1900. No pitcher has ever had a .400+ BABIP across an entire season, and the last time a pitcher even came close in at least 150 IP was in 1872 when Cy Bentley posted one of .385.

So, no, Keller’s ERA has very little, if anything backing it. It also helps that Keller was in the top 69th and 59th percentiles of exit velocity and hard hit rate, and overall average xWOBA, and xSlugging.

Keller has an extremely high ceiling. Not only has he produced excellent numbers at every single level of baseball he has played at, but his pitches are also very enticing. His curveball has the most movement with 59.6 inches of vertical/9.2 inches of horizontal movement. His slider can also be an extremely effective pitch with 35.9 inches of vertical/4.1 inches of horizontal movement. His fastball can touch the high-90’s, but usually sits in the 95-96 range. It also has some very good potential, sitting in the 91st percentile of spin rate. Keller does need to mix up his pitch usage though.

Last year, he used his fastball 59.4% of the time, and used his slider, curve and change up all nearly less than 20% of the time. Although his fastball can be a very good pitch, he should use his other ones more often. According to Baseball Savant, in terms of movement and velocity, Keller is very comparable to 2019 Lance Lynn, which is where I would place Keller’s very high ceiling. Lynn posted a 3.67 ERA, 3.13 FIP, and 1.22 WHIP across 208.1 innings. Most of Lynn’s ERA predictors were similar to Keller’s as well. I already stated his FIP, but his SIERA was 3.83, and just .05 more than Keller’s. However, he did have a much lower DRA at 3.15. It’s not out of the question if Lynn could post a sub-3.5 ERA, as he did have a weak defense behind him, and his ERA dropped to 3.38 over the second half of 2019. Regardless, Lynn still had the 3rd highest fWAR in 2019 at 6.8. Only Jacob deGrom and Gerrit Cole passed him.

In terms of bottom line results, I think Jack Flaherty is the ceiling. Flaherty posted a 3.68 SIERA, and a 3.46 FIP across 196.1 innings. He posted an almost identical ground ball rate (38.3% to 39.2%), K/BB ratio (4.2), and home run rate (1.15). The Cardinals ace posted a bottom line result of a 2.75 ERA and 0.97 WHIP. He also finished 4th in Cy Young voting. However, Flaherty was 3rd in opponent BAbi[ at .242, and had a top-3 defense behind him.

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In terms of a floor for Keller’s rookie season, I think you’re looking at a James Paxton like pitcher. Paxton posted a 3.82 ERA, 3.86 FIP, and 1.28 WHIP across 150.2 innings. Paxton also struck out over 10 batters per 9 innings (11.1) while walking them at a rate right round 3 (3.3 per 9). Paxton carried a slightly higher SIERA (3.93), but it wasn’t much higher than Keller’s. He also posted an identical DRA. Paxton and Keller also generated ground balls at a near identical rate (38.4% to 39.2%). Keller would probably post overall better results, but not by a large margin. He still gave up less home runs, and struck out more batters than the Yankee lefty, but aside from that, Paxton is an apt comparison of what Keller’s floor could be.