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I'm cheating a bit here listing three trades together. However, when it comes to the 1986 World Champion Mets, there were so many important players that came together around the same time and made that team as great as it was. It would be difficult to pick one as more important than the rest.
While Lee Mazzilli was a popular player with the Mets his trade to the Texas Rangers on April 1, 1982 brought Ron Darling to New York along with pitcher Walt Terrell.
On December 8, 1983 the Mets sent utility infielder Bob Bailor and little used pitcher Carlos Diaz to the Dodgers in return for lefty starter Sid Fernandez and infielder Ross Jones.
Ray Knight was acquired by the team on August 28, 1984 when the Mets sent light hitting outfielder Gerald Young, career minor-leaguer Mitch Cook, and infielder Manny Lee to the Astros in return for Knight.
In 1986, Darling went 15-6 with a 2.81 ERA. He was the winning pitcher in Game 4 of the World Series versus the Red Sox. In total he pitched 17.2 innings with a 1.53 ERA in the seven game series.
Fernandez had an impressive 15 year career. In 1986, his record was 16-6 and after a 12-2 start he was selected to participate in the All-Star Game. During the World Series Fernandez pitched out of the bullpen and successfully relieved Darling in the deciding Game 7. He retired seven batters in a row with 4 strikeouts with the Mets trailing 3-0. The Mets went on to win the game and the series.
Knight hit .298 in 1986 and was named the National League's Comeback Player of the Year. During the World Series, Knight was part of that remarkable comeback in Game 6, singling and scoring the winning run. He hit .391 and was named the MVP of the series.