Oakland, Calif. ? Detroit first baseman Sean Casey left the AL championship series opener against Oakland on Tuesday night because of a strained left calf.

Casey will undergo an MRI exam today and said he would miss at least Game 2 but hoped to be able to return when the series shifts back to Detroit this weekend.

“You never want to get hurt in the ALCS,” said Casey, who was wearing a boot on his left leg after the game. “You play your whole life for these games. It’s a little frustrating.”

Casey, batting third in the order, went 1-for-3 with a walk as the Tigers won, 5-1, in Game 1. Game 2 is tonight, and Game 3 will be Friday night in Detroit.

With two outs in the top of the sixth, Casey hit a grounder to shortstop, took one step and then grabbed his leg in pain. He made it about halfway down the line and was thrown out to end the inning.

“I felt it right when I swung,” Casey said. “I really felt a jolt in my calf. That’s why I looked back. It felt like somebody hit me or I got hit by something. Hopefully I just rattled some fibers in my calf and I’ll be all right.”

Manager Jim Leyland and a trainer came out to tend to Casey, who hobbled to the Tigers’ clubhouse after the inning.

Casey said he first felt something in his calf running the bases earlier in the game. He’s had a similar injury before and missed a week but hopes this is less serious.

“I just have to see how it feels tomorrow,” Casey said. “Having that that day off Thursday hopefully we’ll help me get back by the weekend.

Ramon Santiago came in the game at shortstop and Carlos Guillen moved to first base in the bottom half of the inning. The Tigers chose not to put Chris Shelton on the ALCS roster, leaving Guillen as the backup first baseman.