The Mets have traded for Yoenis Cespedes in one of the most exciting deals the team has made in a long time. While Cespedes has played a bit of center field in his major league career, but he played left field exclusively this year and has not played much in center over the past couple of seasons. He is undoubtedly an upgrade to a Mets lineup that has struggled to score runs this year.
To make room for Cespedes, the Mets will have to clear a spot on active roster. The 40-man roster has an open spot because of Jenrry Mejia's recent suspension. On the active roster, the team can continue to platoon Juan Lagares and Kirk Nieuwenhuis in center field and play Cespedes and Curtis Granderson in the corners. Or if keeping Michael Conforto in the lineup is a priority—assuming he isn't optioned to the minors to clear a spot on the roster for Cespedes—one of Granderson or Cespedes could be deployed in center.
That would be a significant defensive downgrade, of course, but the Mets could choose to lean on their very good starting pitching and prioritize offense. Assuming Cuddyer returns relatively soon, he'd present the team with a similar choice, though it wouldn't be tough to find platoon work for Cuddyer in either corner outfield spot. For what it's worth, Terry Collins is saying he's comfortable with Granderson in center once Cuddyer returns.
Getting back to the active roster, though, if Conforto sticks around, Eric Campbell looks like a prime candidate to head down to the minors. Campbell has hit just .181/.307/.282 with a 74 wRC+ this year with the Mets, and all of the positions he can play can be adequately covered, especially now that Juan Uribe and Kelly Johnson are in the fold.