Miller will serve as the team's 29th man in a doubleheader against the Cardinals at Wrigley Field, and he'll start the second game. It will mark his MLB debut.
Miller, 25, had a 4.35 ERA in 26 starts split between Double-A and Triple-A in 2019.
Cubs right-hander Kyle Hendricks will start the opener Monday. That first pitch comes at 4:15 p.m., and the game can be heard on 670 The Score.