Flores, a six-year veteran and non-roster invitee of the Rockies in spring training, was 0-2 with a 5.51 ERA for Triple-A Colorado Springs before having his contract purchased by the major league club.
The 33-year-old Flores has made 265 appearances over his six-year career -- most with St. Louis over the past five seasons. He went 1-0 with a 5.26 ERA in 43 games last season.
The 34-year-old Rusch led Rockies relievers in innings pitched (18 2/3) as the team's long man, going 2-0 with a 6.75 ERA. He allowed six runs and 12 hits in 2 2/3 innings in his most recent appearance Wednesday, a 15-11 loss to Houston. Opposing batters were hitting .398 against him.
"He's a great guy, but we needed to make an adjustment and tighten up some things here," Colorado manager Clint Hurdle said.
Hurdle said that Flores would fill more of a left-handed-specialist type of role. Last season, for example, Flores pitched 25 2/3 innings in 43 games. Hurdle said the team would move an existing reliever into the long-man role to compensate.