Klein made his professional debut in 2021 with the Quad Cities River Bandits of the High-A Central.[7] Over 36 relief appearances, he went 7–1 with a 3.20 ERA, 121 strikeouts, and 44 walks over 70+1⁄3 innings pitched.[8] MLB Pipeline named Klein the Royals' Pitching Prospect of the Year following the season's end.[9] He missed the beginning of the 2022 season due to shin splints.[10] He returned to play in mid-May with the Northwest Arkansas Naturals of the Double–ATexas League as a member of their starting rotation, but was later moved to the bullpen.[11] Over thirty games (three starts), he went 1–1 with a 10.51 ERA, 55 strikeouts, and 51 walks over 43+2⁄3 innings.[12] He returned to the Naturals to open the 2023 season.[13] In mid-June, he was promoted to the Omaha Storm Chasers of the Triple-AInternational League.[14] He was selected to represent the Royals at the 2023 All-Star Futures Game.[15] Over 49 appearances between Northwest Arkansas and Omaha, Klein went 1–5 with a 4.62 ERA, 93 strikeouts, and 39 walks over 64+1⁄3 innings.[16]
On November 14, 2023, the Royals added Klein to their 40-man roster to protect him from the Rule 5 draft.[17] He was optioned to Triple–A Omaha to begin the 2024 season.[18] On April 25, 2024, Klein was promoted to the major leagues for the first time following an injury to Alec Marsh.[19] In five appearances for Kansas City, he recorded a 6.35 ERA with six strikeouts across 5+2⁄3 innings pitched.[20]
On January 21, 2025, Klein was traded to the Seattle Mariners in exchange for international bonus pool space.[24] He was optioned to the Triple-A Tacoma Rainiers to begin the season,[25] where he struggled to a 7.17 ERA with 32 strikeouts and three saves over 22 appearances.[26] On May 31, Klein was designated for assignment by the Mariners.[27]
Los Angeles Dodgers
On June 2, 2025, the Los Angeles Dodgers acquired Klein from the Mariners in exchange for Joe Jacques.[28] He pitched in 14 games for the Dodgers, with a 2.35 ERA in 15+1⁄3 innings.[22] In 22+2⁄3 innings over 20 games for the Triple-A Oklahoma City Comets, he finished with a 5.16 ERA.[26]
After being inactive for the first three rounds of the postseason, Klein was added to the Dodgers active roster for the 2025 World Series.[29] In Game 3, with the Dodgers' bullpen depleted, Klein struck out five batters across four scoreless innings and was credited with the win after the Dodgers secured a walk-off victory in the bottom of the 18th inning. The appearance was the longest of Klein's major league career; he had never pitched more than two innings in any major league game. The Dodgers won the series in seven games.[30]
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