Parnell, 30, has not appeared in a major league game since the start of last season. He entered the 2014 season as New York's closer, blew a save on Opening Day and tore his ulnar collateral ligament during the game. He underwent Tommy John surgery in April 2014 and had been on a rehab assignment this season.
Parnell signed a one-year deal with the team in January worth $3.7 million and avoided arbitration. In 15 minor league appearances Parnell struggled, posting an 11.57 ERA.
In Parnell's long absence, Jeurys Famila has emerged as New York's closer, registering 17 saves and blowing just one with a 1.30 ERA this season.
The Mets placed pitcher Dillon Gee on the bereavement list in a corresponding move following the death of his grandmother. The Mets also activated infielder Dilson Herrera from the disabled list and placed infielder Ruben Tejada on paternity leave.
- Jeremy Woo