GOODYEAR, Ariz. -- Chris Perez couldn't even finish his first bullpen session without trouble.

Now he has to save Opening Day.

Cleveland's All-Star closer will miss at least one month and possibly six weeks with a strained left side muscle he injured throwing off the mound for the first time in spring training. Perez said Sunday he had thrown about 30 pitches Thursday when he pulled up with what he thought was a cramp.

Instead, he strained an oblique muscle.

"Just one of those fluke things," he said.

It's the second significant medical setback in camp already for the Indians, who were ravaged by injuries last season. On Friday, the club announced that center fielder Grady Sizemore will not be ready for Opening Day because of a strained lower back.

Trainer Lonnie Soloff said there's still a chance Perez will be ready for Opening Day on April 5, depending on how he does with treatments.

Asked if Perez would miss the opener against Toronto, Soloff said, "I wouldn't say that."

"We'll have to see how things go and how he responds with his throwing sessions once he gets back on a mound," he said.

Soloff said Perez will need four to six weeks to recover and the hope is the hard-throwing right-hander will be able to pitch in games "toward the end" of the exhibition season.

Perez, who had 36 saves in 40 chances and a 3.32 ERA in 64 games last season, believes he can return in time to start the season closing games for manager Manny Acta.

"Opening Day is not out of the question for me," Perez said. "Four to six weeks is on the long side of when I want to be back out there. Obviously, I have to listen to my body. Now, it's just go out and bang out my rehab."

Soloff said Perez was pushing himself too hard so early in camp.

"His body was clearly not ready for the intensity of that bullpen session," said Soloff, who was asked if he meant Perez was not in shape.

"No," he said. "I'm just saying he wasn't prepared for the intensity of the bullpen session."

Perez later said he was just too aggressive.

"What he (Soloff) means by that is it was the first day, I was going 100 percent," Perez said. "He probably wanted me to go 75 or 50 percent, but that's not who I am. I get work in throwing 100 percent. I'm not going to go throw a bullpen at 50 percent and pretty much just waste a day.

"I was doing what I normally do when I throw a bullpen. It wasn't because I came in out of shape or anything."

Because he's a reliever and not a starter needing to build stamina, Perez will not need as much time to get ready for the start of the season. If there's a bright side to his injury, that's it.

"Definitely a positive," Soloff said.

If Perez winds up not being available for the opener, setup man Vinnie Pestano would be Acta's likely first option to close.

Acta has said there are two bullpen jobs up for grabs this spring.