Cardinals manager Mike Matheny said it was "not a good day" for Westbrook, who will be re-evaluated Sunday.
Westbrook suffered a strained right oblique Sept. 8 and postponed his bullpen session Friday. He is 13-11 with a 3.97 ERA in 28 starts this season.
Carpenter made his season debut Friday and allowed two runs in five innings.
St. Louis holds the second wild card spot by 1 1/2 games over the streaking Brewers.