Though it’s early in spring training for a minor injury, it might not be as early for the Cubs’ Ian Stewart.

Stewart, who had a precautionary MRI exam Friday on the left quadriceps he tweaked running the bases Thursday, is in a time-sensitive, prove-it spring after being released over the winter and re-signing on a non-guaranteed one-year, $2 million contract.

Results of the MRI were not available late Friday.

The contract isn’t unusual for an arbitration-level player. But given manager Dale Sveum’s assertion that third base ‘‘is his job to win, not to lose,’’ any missed opportunity to play during the first half of camp is especially precious to Stewart.

If he doesn’t show enough to win the job by March 16, the Cubs can release him and be obligated to pay only one-sixth of his salary ($333,333). If he’s released after that date but before the opener, it’s $500,000.

Meanwhile, Sveum said Friday that Luis Valbuena has a shot to beat out Stewart for the job regardless of the quad, and Valbuena is in the lineup at third for the Cactus League opener Saturday against the Angels. Valbuena hit a home run off Michael Bowden in the intrasquad game Friday.

Sveum planned to play Valbuena at several other positions until mid-March, assuming regular starts for Stewart, trusting Valbuena’s fielding skills at third and knowing Valbuena would have plenty of time at the position if Stewart doesn’t win the job.

‘‘If things work out that way, then down the stretch [Valbuena will] be playing third base quite a bit,’’ Sveum said. ‘‘But this first half of spring, you’ll probably see him a lot more at other positions.’’