Report: Giants' Lincecum to have MRI on ailing back

San Francisco Giants right-hander Tim Lincecum will reportedly undergo an MRI on Thursday after exiting Game 2 of the World Series with back tightness.

The MRI is expected to focus on Lincecum's lower back and left side, sources tell Jon Heyman of CBS Sports. If the exam returns unfavorable results, the report indicates the two-time Cy Young winner could be replaced on the Giants' World Series roster.

"The pitch before the last one, I felt something tighten up in my lower back," Lincecum said following his exit Wednesday. "I just decided not to go any further."

Lincecum retired the first five Kansas City batters he faced, including two strikeouts, before removing himself from the contest in the eighth inning. It was the first time he appeared in a big league game since the final day of the regular season.

"You get out there in the game-time atmosphere and you don't worry about how long it's been," he said. "You just go out there and compete. Your body taps into what it's comfortable with, and today that's what it was."