Westbrook, 35, is 2-1 with a 1.62 ERA in 39 innings spread across six starts this season. He's succeeding thanks to his 61.3-percent ground-ball rate -- the second-highest in all of baseball -- not because he's walked nearly as many batters (18) as he's struck out (19).
Left-hander John Gast will be called up from Triple-A Memphis to start in Westbrook's place on Tuesday. Closer Jason Motte, who will miss the season following Tommy John surgery, will be transferred to the 60-day DL to clear a 40-man roster spot.
Gast, 24, is 3-1 with a 1.16 ERA and 1.06 WHIP in seven starts for Memphis. St. Louis selected him in the sixth round of the 2010 draft, and Tuesday will be his big-league debut.