OAKLAND -- Giants left-hander Jeremy Affeldt pulled his left hamstring in the eighth inning of Sunday's 3-0 loss at Oakland and left the game two batters later.
Affeldt said he felt discomfort as he dashed from the mound to field a sacrifice bunt by Daric Barton, the first batter he faced.
"It zapped me pretty good," Affeldt said.
Affeldt then yielded an RBI single to Kurt Suzuki and another single to Kevin Kouzmanoff before Dan Runzler relieved him.
Affeldt, who likely will be examined Monday, couldn't gauge the severity of his injury.
"I've never had a hamstring issue, so I don't know what to tell you," he said.
San Francisco's top setup reliever, Affeldt is 2-3 with a 3.12 ERA this season, and has converted two saves in 17 appearances.