"It's been a long process for Tyler," said Red Sox president of baseball operations Dave Dombrowski. "In Spring Training, he got shut down, and it's been a long exploration to find exactly what the problem has been. I'm not an expert on thoracic outlet syndrome, but it's an issue where a lot of other avenues are exhausted before they get to this perspective. I know it was a frustrating situation for him at times, because he'd feel good and then not feel as good."
Matt Harvey and Josh Beckett have also undergone surgery for thoracic outlet syndrome, which involves pinched nerves between the collarbone and first rib. Dombrowski said it's "probable" that Thornburg would have a rib removed during the procedure.
Dombrowski said Thornburg could be Major League ready in an estimated nine months after the surgery.
Thornburg was acquired to bolster Boston's late-inning relief corps after the departures of Koji Uehara, Brad Ziegler and Junichi Tazawa. Thornburg was coming off a stellar 2016 season in which he struck out 90 batters in 67 innings while posting a 2.15 ERA.
"It's buyer beware," Dombrowski said of the trade. "There's no way you could have known this. You just hope you get a healthy player back for next year."
The Sox's bullpen, however, could be getting an additional member in the near future. Right-hander Carson Smith, who had Tommy John surgery last year, threw at Citizens Bank Park on Thursday before he headed back to Boston.
Manager John Farrell said he expected Smith, who hasn't pitched in a Major League game since May 14, 2016, to report to Triple-A Pawtucket on Sunday to begin a rehab assignment.
Stephen Pianovich is a contributor to MLB.com based in Philadelphia and covered the Red Sox on Thursday. This story was not subject to the approval of Major League Baseball or its clubs.