Ritchie will start Sunday's game in St. Louis, Mark Bowman of MLB.com reports.
He's taking over the rotation spot vacated by Hurston Waldrep, who was optioned to Triple-A Gwinnett. Ritchie has made seven starts and three relief outings with the big club this season, turning in a 4.60 ERA, 1.47 WHIP and 42:28 K:BB over 45 innings. Ritchie hasn't handled a normal starter's workload for nearly two weeks and threw 35 pitches in relief Thursday, so he'll likely be on a limited pitch count Sunday.