Known for his control throughout his career, Koosman led the International League in strikeouts in 1967, and in 1968 emerged as the Mets’ lefthanded ace. He was NL Rookie Pitcher of the Year and runner-up to Johnny Bench for Rookie of the Year. He broke club records with 19 wins, seven shutouts, and a 2.08 ERA (all set by Tom Seaver the year before). All are still Met rookie records. He also fanned 62 times (in 91 at-bats), the most by an NL pitcher since 1900.
Seaver and Koosman became one of the league’s top righty/lefty starting combos. As a sophomore, Koosman was 17-9 for the 1969 World Champions, and he beat Baltimore twice in the Series. He overcame arm problems in 1971 and was a hard-luck 14-15 (2.84) for the ’73 pennant winners. Peaking in 1976, he recorded a 21-10 mark. But in 1977, Seaver was traded to the Reds, the Mets deteriorated, and Koosman went 8-20 to tie Phil Niekro for the league lead in losses. Koosman left the Mets as runner-up to Seaver or all-time club leader in ten pitching categories.
Sent to the Twins in a December 1978 trade for Jesse Orosco, Koosman rebounded for a 20-13 record in 1979. In the strike-shortened 1981 season, his 13 losses for the Twins and White Sox were enough to tie for the AL lead. Recurring arm and shoulder trouble ended his career after two seasons with the Phillies.