Mattison (elbow) was optioned to Triple-A Toledo on Sunday, Evan Petzold of the Detroit Free Press reports.
The 25-year-old didn't see action in spring games while working through his rehab from Tommy John surgery. Mattison last pitched at the Double-A level in 2023 and had a 1.62 ERA over 33.1 innings, and he could be ready for action with Toledo at some point this summer.