St. Louis Cardinals pitcher Chris Carpenter grounded out in the fourth inning of Tuesday’s game against Arizona. When he returned to the mound to warm up for the bottom of the inning his left rib cage hurt more with each pitch.

Wednesday morning it was reported that he has a rib cage strain and that he may be forced to make yet another trip to the disabled list.

This has to be frustrating, not just for Carpenter but for the Cardinals, who got just 21 1/3 innings out of their former ace in 2007 and 2008 combined. Still, as upsetting as this setback might be, the Cardinals have to be happy with the signs they’ve seen from Carpenter’s two starts so far in 2009.

In three innings last night, he shut out the Diamondbacks on four hits with two strikeouts. That followed a seven inning, one-hit, two-walk, seven-strikeout performance last week against Pittsburgh in his 2009 debut. That’s 10 shutout innings with a 0.00 ERA, a WHIP below 1.00 and nine strikeouts – it’s a small sample size, but them there are ace numbers.

The Cardinals certainly weren’t putting all their eggs in Carpenter’s basket. This team has Adam Wainwright, Kyle Lohse and Todd Wellemeyer all coming off of solid, double-digit win seasons during which they all sported ERAs below 4.00. Any offense featuring Albert Pujols and supported by solid contributors like Rick Ankiel, Ryan Ludwick and, eventually, Troy Glaus has at least the potential to score runs.

There are also holes. The bullpen is a mix-and-match work-in-progress, with three people sporting saves in the season’s first two weeks – none of which have come from the supposed season-opening closer Jason Motte.

But it’s the starting rotation that is going to determine whether or not the Cardinals can stay in the race with Chicago and Milwaukee all season. And if Carpenter is able to successfully make this comeback from arm problems and two lost seasons it’ll give the team a starting rotation as deep as nearly any team out there.