PHOENIX — The St. Louis Cardinals placed pitcher Chris Carpenter on the 15-day disabled list with a strained left ribcage on Wednesday and said he could be out four to eight weeks.

"If it's not bad, it's four (weeks)," manager Tony LaRussa said before the Cardinals met the Arizona Diamondbacks on Wednesday afternoon. "If it's bad, it's eight. I have no idea. Just keep our fingers crossed."

Carpenter is expected to undergo an MRI exam in St. Louis in the next few days.

Carpenter was injured grounding out to third base to end the top of the fourth on Tuesday night. Carpenter went to the mound to warm up before the bottom of the inning, then called for a trainer.

"I felt it just a little bit on that swing, and every warmup pitch got worse and worse until the last one bit pretty good," Carpenter said after the game. "So there's nothing I could do about it but come out."

St. Louis led 3-0 when Carpenter left the game, but the Diamondbacks rallied for a 7-6 victory in 10 innings.

Carpenter, the 2005 NL Cy Young winner, has not allowed a run in 10 innings this season.

Carpenter is making his ninth trip to the disabled list. The previous eight DL stints were for shoulder and elbow injuries.

"The thing is, he didn't hurt his arm," LaRussa said. "I always like half-full better than half-empty."

The 33-year-old right-hander made only four appearances last season after reconstructive elbow surgery. He underwent another procedure in November to transpose an elbow nerve that was irritating the muscles in the back of his shoulder and around his neck.

The Cardinals also optioned Brad Thompson to Triple-A Memphis. To fill the roster spots, they recalled right-handers Mitchell Boggs and Chris Perez from Memphis, and both pitchers were expected to arrive in time for Wednesday's game.

With Carpenter out of the rotation, LaRussa said the club plans to promote righty P.J. Walters to face the Chicago Cubs in Wrigley Field on Friday.

"It'll be a heck of an assignment," LaRussa said. "He's a composure guy. He's used to being under the gun. We'll see."

LaRussa said Carpenter's loss hurts, but he didn't want his players focusing on the pitcher's absence.

"It's one of those tricky deals where you want to pay respects to what he means to us, which is a lot, but if you keep talking about it, pretty soon the players think, 'Well, we can't win without him,"' LaRussa said. "You've got to find a way to keep competing."