Ferguson Jenkins
A Hall of Fame pitcher (most notably for the Chicago Cubs) who spent nineteen years in the major leagues, Ferguson Jenkins wasn’t bad during his short stint with the Red Sox, but he wasn’t anything close to what he’d been in the years before. From 1967 to 1975 he won twenty or more games seven times (and in the other two seasons, he won fourteen and seventeen games).
Upon joining the Red Sox for the 1976 and 1977 seasons, his win totals plummeted. He went 12-11 in 1976 and 10-10 in 1977, his two worst seasons since becoming a full-time starter in 1967.
That wasn’t an aberration, either, as apart from a bounce-back season in 1978 with the Texas Rangers when he went 18-8 and his 16-9 record in 1979, he was never over .500 again in a season for the remainder of his career.
Jenkins wasn’t terrible during his two seasons in Boston, but if the Red Sox and their fans were expecting him to be the perennial twenty game-winner he had been for the Cubs and Rangers prior to his stint in Boston, they were absolutely let down.