They also recalled outfielder Shane Robinson from Triple-A Memphis and optioned right-hander Carlos Martinez to the minors before a weekend series against the NL Central-leading Brewers.
Masterson (4-6, 5.51) will make his first start Saturday night and Lackey (11-7, 3.60) is expected to face his old team Tuesday in an interleague series against the Boston Red Sox.
Robinson is in his fourth stint with the Cardinals, and was batting .140 with two RBIs in 31 games. At Memphis, he was batting .304 with two homers and 16 RBIs in 53 games.
