The Cardinals selected Mathews' contract from Triple-A Memphis on Saturday.
After posting a 3.13 ERA and 1.14 WHIP through 92 innings at Memphis, Mathews will make his MLB debut Saturday against a Blue Jays lineup that's put up just a .642 OPS since the All-Star break. His promotion gives the Cardinals six starters in their rotation, though the 25-year-old could claim a permanent spot with the big club if St. Louis trades away another starter before the deadline Monday. Matt Svanson was optioned to Triple-A Memphis to make room for Mathews on the active roster, and Tink Hence was DFA'd to clear a 40-man spot.