ST. LOUIS -- Left-hander John Gast will undergo surgery Friday to repair a tear in his left lat, which he said will likely keep him out another four to five months.
Gast was placed on the 15-day disabled list May 26, diagnosed with a left shoulder strain. In his third start of the season May 25, he exited after facing just six batters, citing discomfort in the back part of his shoulder.
Although initially expected to return within a month or two, the 25-year-old rookie said Wednesday that the muscle was progressing "poorly."
"That's obviously why we needed to have surgery," Gast said, "because it wasn't getting better."
Manager Mike Matheny said in late June that he didn't expect Gast to be out so long with the injury, adding that he was still a few weeks away from beginning a throwing program.
"He's still moving along pretty slowly in the rehab process," Matheny said June 22. "But it sounds like it's a pretty complicated injury that does take time."
Gast was called up in May to start for an injured Jake Westbrook and made his Major League debut May 14, a 10-4 win over the Mets. Gast improved his record to 2-0 his second time out before taking a no-decision in his injury-shortened third start.
Chad Thornburg is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.