The team announced Wednesday that Casey, who suffered the injury trying to run out of the batter's box in the sixth inning of Tuesday's Game 1 win, has a small partial tear of the fascia tissue in his calf. He'll receive treatment Wednesday and Thursday, then be re-evaluated on Friday.
Under normal circumstances, manager Jim Leyland said he was told, the injury is around a 10-day absence, which would cost him the rest of the ALCS. While Leyland said they're leaving open the hope that Casey could beat the timetable and return for a possible Game 6 in this series, Leyland said he'd consider that unlikely.
"Because of the type of injury," Leyland said, "they're going to try to speed up the process."
Casey, asked after the game, said he thinks he could be back if the series goes that far.
"I think it's realistic to think I'll have a chance to come back [for a Game 6]," Casey said.
While the news wasn't surprising, it sets in the idea that the Tigers will have to move on for more than a couple of days without their first baseman and No. 3 hitter. Casey, acquired from Pittsburgh at the July 31 trade deadline to provide a left-handed bat, ascended into the middle of the order down the stretch. Though he hit .245 in the regular season with the Tigers, he hit .320 with runners in scoring position.
He was no small part of their AL Division Series win over the Yankees, going 6-for-17 and providing steady defense at first base.
Because rosters can't be changed in the middle of a postseason series, Detroit can't call up backup first baseman Chris Shelton. That leaves them starting Carlos Guillen at first base, a position he played every day for Team Venezuela in the World Baseball Classic and on occasion for the Tigers in the regular season.
One of the Tigers' three utility infielders will start at shortstop. Neifi Perez started there Wednesday, but Leyland said he'll consider it on a day-to-day basis. Part of the reason for starting Perez in Game 2 was that he has better success against A's starting pitcher Esteban Loaiza than Ramon Santiago and for his playoff experience. Leyland said he might play Omar Infante if he needs an offensive spark.
"What happens is you just improvise when you get in situations like this," Leyland said. "So we'll do what we have to do."
As for replacing Casey in the third spot, Leyland made a somewhat unexpected move and shifted Placido Polanco down from the second spot.
"He's a guy you can put anywhere," Leyland said, "and he's not going to change [his approach]."
Jason Beck is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.