ANAHEIM -- Jharel Cotton’s hamstring issue turned out to be worse than anticipated.

Nearly all the way back from Tommy John surgery, Cotton will have to go through another rehab process after the right-hander underwent surgery in Dallas on Thursday with Dr. Daniel Cooper to repair a torn tendon in his right hamstring.

A’s head athletic trainer Nick Paparesta revealed that Cotton sustained a milder hamstring strain last month during sprints as part of his rehab in Arizona, but the injury subsided over 10 days and the right-hander was back to throwing shortly after. After a rehab appearance with Class A Advanced Stockton on May 28, Cotton felt a pop in his hamstring soon after he went out to play catch at the Oakland Coliseum on Friday. MRI results the next morning showed a tear of his tendon that is attached to the bone at his right knee joint.

The surgery was optional, but a decision both Cotton and the A’s felt was in his best interest in order to ensure a full recovery.

“We’re confident this will be able to address Jharel’s problem and get him back at a fairly rapid pace for us,” Paparesta said. “When that might be will be at some point this season. We anticipate Jharel to pitch again.”

Cooper, who is also the head physician for the Dallas Cowboys, performed a right hamstring debridement procedure, which involves the removal of the tendon to allow the hamstring to function without the inflammation caused from the torn tendon.

Cotton will remain in Dallas for a week of rehab and return to Oakland when the A’s return from their current 10-game road trip on June 13.

The surgery is certainly a setback as Cotton was expected to be the first of several injured A’s pitchers to return, but if there is a silver lining here, it’s that the injury will not prevent him from throwing. Cotton will continue throwing in about a week in order to keep his arm moving and is still expected to pitch for the A’s this season out of the bullpen.

“We made the decision it would be best to help Jharel get through this year in a limited role,” Paparesta said. “It is his back leg, his push-off leg, so we wanted to make sure he was confident about it. Jharel felt that, and it was his decision ultimately.”

Mengden optioned

After surrendering a season-high six earned runs in just 2 1/3 innings as the follower to opener Joakim Soria in Wednesday’s 10-9 loss to the Angels, Daniel Mengden was optioned to Triple-A Las Vegas. The move was made partly to add another reliever, Ryan Dull, after the A’s went through six relievers on Wednesday. Mengden was struggling with a 5.09 ERA in five outings.

“This is a new year and obviously he didn’t pitch very well, I don’t know if that affected him,” A’s manager Bob Melvin said of Mengden’s role as the follower. “But when you get a chance to perform, you go out there when you’re called upon and that’s when you do it. He knew ahead of time we were going to do that.”

Melvin said the A’s will call up right-hander Paul Blackburn from Triple-A Las Vegas before Saturday’s doubleheader with the Rangers to start one of those games.