Johnson underwent arthroscopic left knee surgery Monday, Martin Gallegos of MLB.com reports.
It's unclear how long the knee had been bothering Johnson, but he's expected to be ready to roll by the time spring training comes around. The right-handed reliever made 28 appearances between the Phillies and Athletics in 2026, finishing with a 5.88 ERA and 40:20 K:BB across 33.2 frames.