Seattle Mariners to call up prized pitching prospect George Kirby, sources say 1:42 PM ET ESPN The Seattle Mariners are calling up right-hander George Kirby , one of the best pitching prospects in baseball, sources told ESPN's Jeff Passan on Saturday. Kirby, who was scratched from his start for Double-A Arkansas on Saturday, will be making his major league debut whenever he takes the mound for the Mariners. The Mariners on Thursday had optioned young right-handed starter Matt Brash to Triple-A Tacoma, with the plan to transition him to a bullpen role for the rest of the season. The 24-year-old Kirby, who was the 20th overall pick in the 2018 MLB draft out of Elon, has a huge ceiling with excellent command and above-average stuff, including his fastball, which averaged 97.3 mph over the 2021 season. Editor's Picks Kiley McDaniel's top 100 MLB prospects for 2022 81d Kiley McDaniel 1 Related Some Mariners brass have even called him him "robopitcher" because of his excellent control. Kirby has dominated in his five starts in the Double-A Texas League this season, going 2-0 with a 1.82 ERA and 24 strikeouts in 24 2/3 innings for Arkansas. He certainly could provide a boost for the Mariners, who have a 3.68 staff ERA and are in the midst of playing 16 games in 16 days. He was ranked as the No. 34 prospect to watch by ESPN entering the 2022 season. Brash, who turns 24 next week, was 1-3 with a 7.65 ERA in five starts. In other moves Saturday, the Mariners placed catcher Tom Murphy on the injured list with a dislocated left shoulder and recalled catcher Cal Raleigh from Triple-A Tacoma. Murphy suffered the injury in the fifth inning of Friday's game and is batting .303 with a home run and nine runs scored in 14 games this season. ESPN's Kiley McDaniel contributed to this report.