On February 8, 1959, the Red Sox announced that they had Sisler signed for the 1959 season. After what was considered a "disappointing" season, he did not impress with his first outing in spring training, giving up two hits and five bases on balls in just two innings of work. Lack of pitching depth was a noted weakness for the Red Sox entering the season, with unpredictable youth and with hopes that Sisler could regain his previous level of success. Sisler opened the season slated as a relief pitcher with the idea that he would start in certain cases. In three relief appearances through April, he had a 6.75 ERA in 6⅔ inning pitched. Management decided that his performance along with a pitching staff that they considered inexperienced, Sisler and Ted Lepcio were traded to the Detroit Tigers on May 2 for starting pitcher Billy Hoeft.