Zimmermann is starting Game 2 of Thursday's doubleheader against the Rays, Joe Trezza of MLB.com reports.
The roster move won't be official until after the completion of the matinee, but the 25-year-old is penciled in as the starter for Game 2. Zimmermann made his way to Triple-A in 2019 and made seven starts with a 4.89 ERA, 1.62 WHIP and 33:18 K:BB over 38.2 innings. According to Roch Kubatko of MASNSports.com, the young right-hander may not remain in the rotation after Thursday, but he'll still have a role with the big-league club for the closing stretch of the season.