1990 Mariano Rivera

Mariano Rivera came to the big leagues as a starting pitcher. Since he became a relief pitcher at the end of the 1995 season, he's put up a 1.87 career ERA over the regular and postseason.

In his 5+ seasons in the minors, he had 9 different pitching assignments (by that I mean some years he pitched at more than one level), and in those nine assigments, only in 1994 at AAA did he have an ERA over 2.75.

In all of those assigments he was a starting pitcher though, except for one of them. His very first assigment with the Yankees was as a relief pitcher in rookie ball in 1990. Anybody playing in that rookie league would not be surprised that Mariano Rivera would go on to become a hall-of-fame relief pitcher. In 52 innings he gave up just 1 earned run, for a 0.17 ERA.

In 1991 he became a starting pitcher, and would stay that way until he was put back into the bullpen as a rookie in 1995.