Lynn joins pitchers Adam Wainwright and Jordan Walden, first baseman Matt Adams and outfielder Matt Holliday among Cardinals currently on the disabled list.
Lynn last pitched on Sunday against the Dodgers in Los Angeles, allowing two runs in five innings and making 98 pitches, and he felt discomfort in his right forearm in that start. He left the club Tuesday to return to St. Louis for further examination.
Lyons, 27, previously made three starts with the Cardinals from May 5-16. He was 3-1 with a 2.36 ERA over his last four starts at Triple-A.