Mize, 25, has been on the injured list with a right elbow sprain since April 15 after making two starts this season.
Mize's ulnar collateral ligament in his elbow is still intact, but Dr. Keith Meister determined the ligament had stretched to the point where it had lost elasticity and functionality.
"I'm very sad for Casey, for the work he put in and for the ultimate diagnosis," Hinch said.
The date of surgery has not been determined.
Mize got two outs for Triple-A Toledo on May 12, then had his rehab program slowed down. Hinch announced on Tuesday that Mize had been shut down due to continued elbow discomfort while rehabbing at the team's spring training facility in Lakeland, Florida.
Mize has a 7-13 record with a 4.29 ERA in 39 big league appearances, all of them starts.