He will have an MRI on his lower back/side on Thursday, Jon Heyman of CBS Sports reports.
Lincecum, who had pitched the seventh inning, faced five batters in the eighth and threw 23 pitches before leaving the game after short-hopping a pitch to Royals catcher Salvador Perez with two outs. The Royals won the game, 7-2, tying the best-of-seven series at one game apiece.
Game 3 is Friday in San Francisco. Jeremy Guthrie will take the mound for Kansas City and Tim Hudson gets the start for the Giants.
It was Lincecum’s first appearance during this postseason after spending the NLDS and NLCS on the bench. Before last night, his previous appearance had been on Sept. 28.
"It was just the pitch before the last one; I felt something tighten up in my back," Lincecum said, according to MLB.com. "I just decided not to go any further."
Lincecum will receive treatment on Thursday. He did not say if he would be ready to pitch on Friday.
"We're just going to treat it, see how it feels tomorrow, but right now it just feels pretty stiff," Lincecum said. "I think we're just going to play it by ear, take it day by day and see how it feels tomorrow."
Lincecum had 12-9 record with a 4.74 ERA during the regular season, which included his no-hitter against the San Diego Padres in June.
Giants manager Bruce Bochy said he liked the way Lincecum threw the ball against the Royals and will use him during the Series as long as he is healthy.
"Oh yeah, he threw the ball well, he did," Bochy said. "I mean, we're going to need help in the sixth and seventh innings. I like the way he threw the ball today."
- Scooby Axson