During spring training in 1973, the "Sarasota Herald-Tribune" labeled Stein as the Cardinals candidate for pinch hitting off the bench. He made the Cardinals Opening Day roster that year. He made his season debut on April 6 as a pinch hitter, going hitless in one at-bat against the Pittsburgh Pirates. His first hit of the season came on April 17, against the Pirates. In August, Stein was sent down to the minor leagues and was replaced on the Cardinals major league roster by outfielder Héctor Cruz. In the minors, he played with the Triple-A Tulsa Oilers, where he batted .289 with 23 hits, two doubles, and one triple in 21 games played. While in the majors that season, Stein compiled a .218 batting average with four runs scored, 12 hits, two doubles, and two RBIs in 32 games played. On defense with the Cardinals, he played right field, left field, third base, and first base.