CHICAGO -- Approximately 10 pitches into his scheduled bullpen session on Saturday morning, Jake Westbrook stopped throwing. Pitching coach Derek Lilliquist and manager Mike Matheny walked his way.

The meeting on the mound was brief, and it ended with Westbrook dropping the ball and walking back into the clubhouse. His right oblique muscle was bothering him again, enough so that Westbrook said shortly after that "it's hard to image" he'll be able to pitch again this season.

Westbrook strained his right oblique near the end of a start against the Brewers on Sept. 8. His recovery seemed quick, so much so that Saturday was supposed to be Westbrook's last hurdle before returning to the rotation.

It turns out that the right-hander hadn't fully healed. And now he will be sidelined for an indefinite amount of time.

"It was just there," Westbrook said of the discomfort. "Knowing that I feel it then in the first 10 pitches, when I start getting after it, it's not going to be much better. It has slowly gotten worse each day. I was hoping I could bounce back. That's not the case."

With only 12 days until the end of the regular season, Westbrook does seem short on time to work toward a regular-season return. The Cardinals are at least fortunate that Westbrook's shutdown coincides with Chris Carpenter's return. As a result, the rotation will stay on its current schedule.

That means Lance Lynn will start Monday's series opener in Houston. Jaime Garcia will pitch Tuesday, followed by Carpenter on Wednesday. With an off-day Thursday, the Cardinals can then fiddle with their rotation for the final homestand, if they so wish.

Westbrook was in the midst of one of the best seasons of his career before the oblique injury. If his season is indeed over, Westbrook will finish 13-11 with a 3.97 ERA in 28 starts. That's the most wins Westbrook has had in a season since 2006.

"It's terrible timing," Westbrook said of the injury. "That's what's so disappointing about it."

Cardinals bullpen not at full strength

CHICAGO -- A day after having to piece together a bullpen without setup man Mitchell Boggs and closer Jason Motte, Cardinals manager Mike Matheny was expected to have other relievers unavailable for Saturday's game.

While Matheny rarely announces that someone is off limits for competitive reasons, it's hard to envision him calling upon Edward Mujica or Shelby Miller for the second game of the three-game series against the Cubs.

The concern with Mujica is workload-related. The right-hander, who admits he's feeling some fatigue, has appeared in six of the team's last eight games. He is one appearance away from matching his career high of 67.

Miller hasn't been burdened with a heavy workload, but his inexperience as a reliever factors into how Matheny uses him. Having ascended through the Cardinals' farm system exclusively as a starter, Miller has never pitched on consecutive days. In fact, none of his four Major League relief appearances have come with fewer than four days off.

Miller threw 13 pitches in an appearance on Friday.

"We're going to be careful with a young player like that who hasn't been exposed to back-to-back days," Matheny said. "It's getting accustomed to the whole routine of what a reliever does. There's a progression, typically."

The Cardinals' conundrum in the back end of their 'pen is the result of playing so many close games. Ten of the team's 19 games this month have been determined by one run. Only four of those contests have finished with a margin of victory greater than three.

As a result, Mujica, Boggs and Motte have already been called upon to make a combined 31 September appearances.

"That's not necessarily how we want it to line up, but we've been in a lot of close games," Matheny said. "Every one of them it seems like it's one play away, one pitch away from positive, negative. It shows the team is competing. We're right there. And when you're right there at the end of the season, you have to take your best shot."

Worth noting

• Joe Torre, who serves as the executive vice president of baseball operations for Major League Baseball, credited Carlos Beltran with a second-inning single in the Cardinals' Sept. 14 game after being asked to review the call made by the official scorer. That official scorer at Dodger Stadium initially ruled an error on shortstop Hanley Ramirez on the play.

• As of Saturday, the Cardinals had not been contacted by Major League Baseball about changing the game time for their final game of the season. The D-backs announced on Friday that they will start earlier than scheduled on Oct.3 per a request from MLB to have games end earlier because of the potential of tiebreaker scenarios.

The start time for St. Louis' Oct. 3 game against the Reds is 7:15 p.m. CT.