Cardinals right-hander Chris Carpenter will start an official minor league rehab assignment with Double-A Springfield on Monday, reports Derrick Goold of the St. Louis Post-Dispatch. Pitchers get a 30-day rehab window.

Carpenter, 38, was not expected to pitch this year due to lingering neck, arm and shoulder issues. Retirement was considered a possibility. He started a throwing program in May and has gradually continued to get back into pitching shape, which included throwing a simulated game at Busch Stadium on Wednesday.

Injuries limited Carpenter to only three starts last year, during which he went 0-2 with a 3.71 ERA. In 2011, his last full season, he went 11-9 with a 3.45 ERA before a dominant postseason showing that led the the Cardinals to the World Series title.

St. Louis has the lowest rotation ERA (3.22) and WHIP (1.15) in baseball this season. Both Shelby Miller and Adam Wainwright are having Cy Young caliber seasons while Jake Westbrook and Lancy Lynn have been rock solid, but Jaime Garcia is out for the year with a shoulder issue. Joe Kelly has gotten the latest look as the team's fifth starter.