Feliz, 24, won the 2010 American League Rookie of the Year award as the Rangers' closer and was moved into the starting rotation at the beginning of the season.
But after going 3-1 with a 3.16 ERA in eight games with 37 strikeouts and 23 walks, he was placed on the disabled list May 21 with elbow soreness.
A minor-league rehab start scheduled for the weekend was canceled and a subsequent examination revealed a torn ligament in the right-hander's throwing elbow.
"It's a blow for him and for the club," he told ESPN. "Doctors feel he should be a perfect candidate to have a 100 percent recovery and be back at some point next summer assuming all goes well."