Each team announced its starting pitcher on Tuesday.
Keuchel, who has nine career postseason starts, went 8-8 with a 3.75 ERA in 19 starts after signing a one-year deal with Atlanta. He will face the Cardinals for the first time since 2016.
Mikolas was 9-14 with a 4.17 ERA in 32 starts for the Cardinals. He will make his postseason debut.