Mariners Promote Top Prospect George Kirby To Make MLB Debut

The Mariners top pitching prospect will look to help turn around Seattle's season after a slow start.

The Seattle Mariners are looking to inject some life into their starting rotation, as top pitching prospect George Kirby will make his MLB debut on Sunday.

Kirby was a first-round pick back in the 2019 MLB Draft out of Elon University. The 24-year-old right-hander has pitched to a 2.34 ERA across 115 1/3 innings pitched across his minor league career.

Last season, Kirby pitched to a 2.53 ERA in 15 starts split between High-A and Double-A. He began this season back in Double-A, where he pitched to a 1.82 ERA with 32 strikeouts in 24 2/3 innings before the Mariners decided they had seen enough and elected to promote him.

Chris Flexen was supposed to start on Sunday, but will now have his start pushed back to Monday instead. Meanwhile Kirby is taking Matt Brash’s spot in the rotation, as the 23-year-old was sent down to Triple-A on Thursday.

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Brash made five starts for the Mariners this season and pitched to a 7.65 ERA with a 2.05 WHIP. Seattle plans to transition Brash into a reliever role for the remainder of the season, where they hope to see his stuff play up a bit out of the pen.

Considering Kirby’s status as one of the top prospects in baseball, with one of the highest floors, the hope is that he will become a mainstay in the Mariners rotation for years to come.

Scouting Report

In our latest updated top 100 prospect list, George Kirby checked in at No. 20, making him the sixth-best pitching prospect in all of baseball. The following is Aram Leighton‘s breakdown of Kirby as seen in the top 100.

20.George Kirby – RHP – Seattle Mariners

Age: 23 | Height/Weight: 6’4′, 215 | Bat/Throw: R/R | 1st Round (20), 2018 (SEA) | ETA: 2022


2021 Stats A+/AA: 67.2 IP, 2.53 ERA, 2.37 FIP, 1.08 WHIP, 29.2 K%, 5.5 BB%


High ¾ arm slot with a short arm action. Fastball has been 90-94 in the past, but was reportedly touching the upper 90’s at the Mariners alternative camp. It has some arm-side run and sink, but the added velocity is huge for his long term value, as his fastball can become hittable at times. With that being said, a version of Kirby sitting in the mid 90’s is terrifying given his incredible command of the pitch. He can robotically locate it to his glove side, even better than most major leaguers. He can also locate it to his arm side at an above average clip, giving his fastball command a rare chance of being elite.

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Kirby’s curveball is his best secondary offering, flashing plus with it’s lower 80’s velocity. It has hard, late bite with it’s 11-5 action, and it’s his go-to pitch for whiffs. He can already bury it for strikeouts, as well as throwing it for strikes at will.

His slider gives him another quality breaking ball despite it’s lack of current depth. It sits in the mid to upper 80’s and has solid horizontal movement without the expense of much drop. Like the first two pitches in his arsenal, he can throw it for strikes whenever he wants. He uses it to steal strikes and produce weak contact against right handed hitters, but as of now it doesn’t project to miss a lot of bats.

The changeup is the only pitch in his arsenal that lacks average current command, but the drop and arm-side run on it give it the potential to be another above average offering. It sits in the mid 80’s, and he uses it mostly against left handed hitters in even counts. As he develops the pitch, it could become a soft-contact inducing machine, capable of producing lots of ground ball rollovers.


Kirby is the rare type of prospect who has both a high floor and a high ceiling. His increased fastball velocity combined with his potentially elite command gives him the floor of a third or fourth starter capable of eating innings while excelling at producing soft contact. If he’s able to further refine his changeup and slider, Kirby has the potential to have four distinct speed differences, as well as multiple weapons to both induce soft contact and miss bats. If he’s able to accomplish this, Kirby has the ceiling of a number two starter, and an outside chance of becoming an ace.