OAKLAND, Calif. (AP) – Detroit Tigers first baseman Sean Casey has a partial tear in a muscle in his left calf and could miss the rest of AL championship series against the Oakland Athletics.

Casey injured himself in the sixth inning of Detroit’s win in the series opener and underwent an MRI exam Wednesday that showed the small tear.

“The trainers are hoping for a miracle when they say possible by the end of this series,” manager Jim Leyland said before Game 2. “But I think that’s probably not going to happen. So I think we’ll just have to do with what we have.”

General manager Dave Dombrowski was more optimistic, saying there was “a good possibility” Casey could return late in this series and an even better chance he’d be able to play if Detroit made it to the World Series.

The injury could be a costly one for Detroit, which does not have another true first baseman on its ALCS roster after not activating Chris Shelton. Shortstop Carlos Guillen moved to first base after Casey left Game 1 and started there Wednesday night. Neifi Perez started at shortstop.

Guillen played eight games at first base in the regular season this year – his first major league action there in his career. He also has played first base in the World Baseball Classic and in winter ball in Venezuela.

“To me it’s easy,” he said. “I don’t feel it’s too hard. Shortstop is hard. When you play shortstop, you can play everywhere.”

Guillen made his first error at first base Tuesday on Jason Kendall’s spinning grounder in the seventh. He made up for it a batter later by starting a 3-6-1 double play.

The injury also forced Leyland to juggle his lineup, moving Placido Polanco down a spot to the No. 3 hole and plugging Perez into the No. 2 hole.

“What happens is you just improvise when you get in situations like this,” Leyland said. “We’ll do what we have to do. Is it an ideal situation? No it’s not. But I remind everybody that the Oakland A’s lost their double play combination so we’re certainly not looking for any sympathy, nor do we expect any.”

Oakland is playing without injured shortstop Bobby Crosby and second baseman Mark Ellis in the ALCS.

Casey batted .245 with five homers and 30 RBIs in 53 games after being acquired in a trade from Pittsburgh. He is 7-for-20 with four RBIs in the postseason.

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