The 31-year old right-hander said Saturday the operation likely will be performed by Dr. James Andrews, probably next week.
"After talking with the doctors here and going down to see Dr. Andrews, he confirmed what the doctors here thought," Hanrahan said in Boston's clubhouse. "I'm going to have to get surgery to repair my flexor tendon. It's something that, after hearing the doctors here, it's something I kind of prepared for. He just reaffirmed those. I'm going to get it done next week and start the process of getting healthy."
Hanrahan walked off the mound in the ninth inning of Monday's 6-5, 11-inning win over Minnesota after throwing a pitch.
Obtained from Pittsburgh in an offseason trade, the two-time All-Star is making $7.04 million this year and is eligible for free agency after the World Series. He is 0-1 with a 9.82 ERA in nine games and was on the disabled list from April 14 to May 7 with a strained right hamstring.
During his brief stint with the Red Sox, he conveted four of six save chances. Now, he's hoping that it's only the forearm that needs surgery—and not a elbow ligament-replacement operation, as well.
"He'll look at that when he's in there. The MRI said it looked pretty good," Hanrahan said. "Nobody wants to just do Tommy John for fun. He's going to go in and look and see if that's something that needs to be done or not. As of now, it doesn't need to be. But once he gets his eyes on it, he'll have a better idea."