Westbrook spent time on the disabled list earlier in the year with an elbow injury. The veteran struggled in 12 starts since he returned from his first stint on the DL, going 5-7 with a 5.97 ERA, only striking out 24 batters while issuing 30 walks. His struggles worsened in August, surrendering 26 runs in 21 innings over three starts and one appearance out of the bullpen.
Martinez spent some time with the Cardinals earlier this year, appearing 10 times out of the bullpen and making one start. The rookie posted a 6.91 ERA over 16 innings, but owns a 2.66 ERA as a starter in 12 starts at Triple-A.
Westrbrook was scheduled to face the Cincinnati Reds on Monday, but no pitcher has been officially announced to replace him. Michael Wacha, Tyler Lyons and Martinez are all candidates on the 25-man roster who could start against the Reds.