Mack won't be available for training camp. His injury clears the path for Notre Dame transfer Brandon Wimbush to claim the starting job.
The Knights are already without star quarterback McKenzie Milton as he recovers from a gruesome leg injury. Milton will likely take a medical redshirt this season to retain his final year of eligibility for 2020.
Mack took over after Milton's injury, starting the AAC championship win over Memphis and the Fiesta Bowl loss to LSU, UCF's first defeat since the 2016 campaign. The young passer completed 51 of 100 attempts for 619 yards, three touchdowns, and one interception, adding 337 yards on the ground.
UCF opens its 2019 season Aug. 29 versus Florida A&M.