The Royals announced Thursday that Brentz recently underwent Tommy John surgery on his left elbow.
The 27-year-old has been sidelined by an elbow strain since late April, and he began a minor-league rehab assignment in mid-June only to be pulled after one appearance. Given the timing of the procedure, Brentz is likely to miss the entirety of the 2023 campaign in addition to the rest of this season.