Denny had an excellent curve and changeup, but only an average fastball. Control was all-important for his success. He helped himself considerably with fine fielding and passable batting for a pitcher. A temperamental sort, he suffered from weak ankles throughout much of his career.
A hot and cold pitcher, he led the NL in ERA with the Cardinals in 1976 but was only 11-9 on the season. Two years later, he won 14, but when he fell off to 8 wins in 1979, St. Louis traded him to Cleveland. Late in 1982 the Indians dealt him to Philadelphia.
Denny had by far his best season in 1983 with the Phillies. His 19-6 mark gave him league highs in wins and percentage. His 2.37 ERA was his personal best. The Phillies won the pennant, and Denny received the Cy Young Award.