Manager Jerry Manuel said Thursday the move "strengthens us in the bullpen" for the last two weeks of the season.
Manuel said Parnell will return to a starting role in winter ball, but the team's long-term plans for the right-hander are uncertain.
Parnell gave up seven hits and four runs, including three earned, with three walks and four strikeouts in 3 1/3 innings Wednesday night. He got a no-decision in a 6-5 loss to Atlanta.
Parnell is 3-8 with a 5.60 ERA in 62 games, including 1-5 with a 7.93 ERA in eight starts.