Head athletic trainer Dave Groeschner immediately came out to check on the right-hander, who was pulled after completing 1 2/3 innings. The Giants announced he was diagnosed with a right forearm contusion and X-rays were negative.
"The expectation just has to be day by day," Lincecum said. "I think it's going to be sore for about a few days here."
Lincecum was not effective to begin with, allowing three runs on four hits while walking two batters in the second shortest start of his career. His shortest start came in his previous outing in Los Angeles.
"It wasn't a very good job of executing my pitches down in the zone," Lincecum said.
At 31 years old, Lincecum has seen a steady decline in performance. The two-time Cy Young Award winner and four-time All-Star hasn't posted an ERA below 4.00 since 2011.
The right-hander's velocity is the lowest in his career, according to Fangraphs. Lincecum is striking out fewer batters per nine innings and walking more batters per nine than at any point in his career.
With Matt Cain and Jake Peavy set to return to the starting rotation next week, it's possible Lincecum's spot in the rotation is in jeopardy. Moving him to the bullpen remains a possibility, and it would allow him to throw harder with having to work fewer innings.
"These are things we are going to talk about," manager Bruce Bochy said. "His last couple starts, he just has not been on top of his game."
Bochy would not confirm any decisions regarding the rotation following Saturday's game, but said he's worried about his former ace.
"His stuff is down a little bit," Bochy said. "I'm a little concerned, to be honest."
Oliver Macklin is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.