He doesn't expect to miss much time.
"I felt it grab, and it progressively was getting tighter and tighter there," Triggs said. "As far as finishing pitches, it wasn't getting where I wanted it to be. It's just frustrating to put the bullpen in a situation like that.
"I'm hopeful it's something that will be resolved pretty quickly here. Ideally, it would be cleared up fairly soon."
Triggs allowed one unearned run and threw 24 pitches in the first inning before being replaced by right-hander Zach Neal, who was tagged for five runs (four earned) in just 3 2/3 innings.
The injury is the latest in a string of health issues for the A's, particularly their rotation. Just Monday, left-hander Sean Manaea was forced out of a start because of back discomfort, joining five other starters on the disabled list, including Sonny Gray (forearm).
"At this point in time in the season, with what we've gone through, you certainly don't want to push him," A's manager Bob Melvin said. "Hopefully it's just a few days and we can get him back out there pretty soon."
Jane Lee has covered the A's for MLB.com since 2010. Follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.