Cincinnati had the NL's worst bullpen in the second half of last season, when the relievers went a combined 1-17. The Reds signed Gregg to a minor league deal in the offseason, and he made the team out of spring training.
Gregg, who got a $1.5 million major league contract, went 0-2 with a 10.13 ERA in 11 relief appearances, giving up 12 earned runs and three homers in 10 2/3 innings. He gave up a tying two-run homer to Pittsburgh's Andrew McCutchen on opening day and allowed runs in seven of his 11 appearances.
"The fans never got to see the best of Kevin," manager Bryan Price said. "We felt in the six weeks that we had him in spring training that he was the guy who could pitch the eighth inning for us.
"Unfortunately, with the two-run homer on opening day to tie the game, I think that was very difficult. Sometimes, it's hard to chase that off. He was doing everything he could to try to get back on top and it just didn't happen."
The Reds called up right-handed reliever Pedro Villarreal from Triple-A Louisville. He has no record and a 10.13 ERA in two major league appearances this year.
Cincinnati's bullpen was last in the NL with a 5.56 ERA on Monday. The Reds' other main offseason acquisition -- right-hander Burke Badenhop -- was 0-1 with an 11.17 ERA in 11 relief appearances.