The Cardinals called up Cooper Hjerpe from Triple-A Memphis on September 6, 2026, and the left-hander is set to make his major league debut on Sunday. He will follow right-handed starter Kyle Leahy against the Rockies.
The move gives St. Louis its first major league look at a 25-year-old pitcher it took 22nd overall in the 2022 draft, one whose path has been slowed by elbow surgery, a Tommy John procedure and the kind of control problems that can stall a fast-rising arm. Hjerpe was added to the 40-man roster in late November to keep him out of the Rule 5 draft, and he is now on the brink of the majors after spending time on the injured list and in the minors.
Hjerpe’s performance in Memphis is why the promotion matters now. He posted a 0.95 ERA in his first 19 Triple-A innings and struck out 34.2% of hitters there, numbers that explain why the Cardinals finally moved him up. But the same sample also shows the reason for caution: he walked 21.5% of hitters in Memphis, a rate that can turn a promising outing into a stressful one in a hurry.
That split mirrors the larger story of his development. Through 2024, Hjerpe put up a 3.37 ERA and a 32.8% strikeout rate in 93 1/3 innings between High-A and Double-A, while walking about 14% of hitters from 2023 to 2024. He then missed all of 2025 after Tommy John surgery, returned to post a 3.00 ERA in 42 combined innings at three levels, and was ranked as the Cardinals’ 18th-best prospect by MLB Pipeline.
Sunday gives the Cardinals a clean test of whether the strikeout stuff can survive against major league hitters without the walks taking over. For Hjerpe, it is the moment his minor league line has been pointing toward since the organization kept him on the roster last winter; for St. Louis, it is a chance to see whether a comeback arm can turn promise into outs when the pressure finally changes.
