This will be Gaines' second Tommy John surgery, as he had the operation performed on the same elbow when he was 15 years old.
"It is a sad, sad deal," Fox said. "He had that surgery as a ninth grader ... there was a 50-50 chance that he would do it again. That happens when you are young.
"Mark my words - that kid will be back out on the mound for us. He is that dedicated and he has that kind of will and toughness."
Gaines, a native of Weddington, N.C., started seven games this season with a 4-0 record and a 4.08 ERA.