Colorado Rockies long reliever Darren Oliver
Left-handed reliever Darren Oliver spent parts of 20 seasons in the majors from 1993 through 2013 both as a starter and a reliever. However, he was a starting pitcher for his lone season in a Rockies uniform as he made 33 appearances (32 starts) for the Rockies in 2003.
Oliver made 766 MLB appearances (229 starts) with a 4.51 career ERA, a 104 ERA+, and 22.2 rWAR.
Colorado Rockies middle reliever Jeff Fassero
Left-handed reliever Jeff Fassero primarily pitched with Montreal in his 16 MLB seasons and he spent most of the 2004 season with the Colorado Rockies. Like Oliver, he was a starter and reliever but he did both frequently for the Rockies as he made 40 appearances (12 starts).
Fassero made 720 career MLB appearances with 242 starts with a 4.11 ERA, a 107 ERA+, and a career 23.7 rWAR. He also came in 6th in NL Cy Young voting in 1996 with Montreal.
Fassero is the “middle reliever” for the Rockies here since Oliver made more starts with the Rockies but both would really be “long relievers.”
Colorado Rockies middle reliever Kent Mercker
Left-handed reliever Kent Mercker spent 18 seasons in the majors from 1989 through 2008 as both a starter and reliever. However, when he was with the Rockies in 2002 and he was used exclusively in relief in his 58 appearances.
He made 692 appearances (150 starts) in the majors with a 4.16 ERA, a 104 ERA+, and a career 12.0 rWAR.