Art Fowler.
Not exactly an ace at the age of 43 when he joined the Denver Bears as a pitcher/coach, but he was a fan favorite. Led to his friendship with Billy Martin and a long, successful career as a pitching coach.
From Wikipedia:
In 1965, he signed with the Triple-A Denver Bears as a pitcher-coach, and between 1965–68 and in 1970 he worked in a total of 211 games pitched and compiled a 27–15 won-lost record. On May 27, 1968, Billy Martin became manager of the Bears, and he and Fowler began a long friendship and professional association. Fowler served as Martin's pitching coach with the Minnesota Twins (1969), Detroit Tigers (1971–73), Texas Rangers (1974–75), Yankees (1977–79, 1983, 1988), and Oakland Athletics (1980–82). Under his tutelage, Ron Guidry won the Cy Young Award in 1978.