As you have undoubtedly heard over the past few days, Mets pitching prospect Rafael Montero is set to make his debut tonight in Queens against the Yankees. The Mets made room for him on their active roster by placing Dillon Gee on the disabled list, but they also had to clear a spot on their 40-man roster and did so by moving Bobby Parnell from the 15-day disabled list to the 60-day disabled list. Unlike players on the 15-day disabled list, players on the sixty do not count against the 40-player maximum.
So the Mets still have a full 40-man roster, which means they would have to remove a player from it to add a player like Noah Syndergaard who's not already on it. You can ready plenty more about Montero in Jeff Paternostro's primer from earlier today. As for Parnell, he underwent Tommy John surgery after just one appearance and is not expected back until the beginning of next season at the earliest.