Blanton could be out three to six weeks. He suffered the injury while throwing a side session Wednesday.
Phillies team doctor Michael Ciccotti says Thursday if all goes well, "we're looking at the earliest at the third week of April."
Blanton, who hasn't been on the DL in six seasons, was projected as the No. 3 starter behind Roy Halladay and Cole Hamels.
Blanton's most recent spring outing came in a minor league intrasquad game Monday, when he allowed 11 runs and 11 hits in five innings.
Blanton was 12-8 with a 4.05 ERA last season. He signed a three-year, $24 million contract extension over the winter.
