The move is retroactive to Aug. 22. Westbrook is 7-8 this season with a 4.49 ERA in 19 appearances.
In Westbrook's last start, against the Milwaukee Brewers on Aug. 21, he failed to make it out of the fifth inning, giving up nine hits and five earned runs in 4.2 innings.
Westbrook has a 5.97 ERA in 13 games since returning from the disabled list earlier this year for elbow problems. He has allowed 26 runs in 21 innings so far in August.