Leyland said after the win that Casey will not be available for Game 2 of the series in Oakland, and cold weather in Detroit over the weekend could make him doubtful for a return then.
"We don't really know the extent of [the injury] yet, but it's 100 percent that he will not play [on Wednesday]," Leyland said. "Friday is probably a long shot but a possibility, but with the reports of the weather in Detroit, I don't know if that will be feasible or not, but we'll just have to play it by ear."
The injury occurred when Casey hit a grounder to A's shortstop Marco Scutaro in the sixth. It was during his swing that Casey said he felt a pop in his leg, and the pain was so intense that he stopped before even taking off out of the batter's box to see what had happened.
"I looked back because I thought I got hit with a bat or something," Casey said. "I thought I hit myself with a bat. It felt like [A's catcher Jason] Kendall jumped up and punched me -- it was that kind of pain."
After turning to look at his leg, Casey then stumbled out of the batter's box and hobbled down the first-base line, stopping midway when the throw beat him to first. Leyland and a team trainer immediately met Casey on the field, and it was obvious that Casey's left leg was bothering him.
Casey was diagnosed with a strained left calf muscle, and he will undergo an MRI on Wednesday morning to see if there is a tear or a pull in the muscle.
Casey said that he started to feel twinges in his calf two innings earlier after he walked in the third and had to run to second base, but he underwent some treatment and it started to feel better before the final swing in the sixth that really injured the muscle.
Walking around the visiting clubhouse at McAfee Coliseum on Tuesday night with his left foot in a boot, Casey spoke with hope of returning for the weekend games in Detroit. That might be a little bit of wishful thinking, as Casey said that he experienced a pain similar to this a few years ago and missed a little over a week.
"Hopefully, that's not the case this time and it's a different injury," Casey said. "I know I felt something and it wasn't just a cramp, but we'll have to wait until [Wednesday] and see how it feels."
Casey is the only true first baseman that the Tigers have on their roster. Upon Casey's removal from the game, Guillen moved from shortstop to first and Ramon Santiago took over at short.
"We'll have to go with the combination of Santiago and [Neifi] Perez and [Omar] Infante [at shortstop] and Guillen at first base, and we'll have to get through it the best we can," Leyland said.
Guillen saw action at first base during just eight games this season with the Tigers, but he said that he played the position during winter ball and the World Baseball Classic last spring.
"Wherever they need me, I'll be there," Guillen said. "It's not going to be the first time I've played first base, so I feel pretty comfortable over there."
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The Tigers will try to carry on without their steady first baseman, but it's clear that Casey himself is somewhat disappointed with the timing of his injury.
"You never want to get hurt right now, in the ALCS, because this is what you play your whole life for -- to play in these games," Casey said. "So for that reason, it's a little frustrating."
But while Casey is disappointed to be out for at least one game, he was certain that his team would carry on just fine in his absence.
"Carlos will do well wherever he's at, whether it's at first or short, and Ramon or Omar or Neifi will fill in at short, too," Casey said. "I'm sure those guys will be all right."