The Mets have a five day window after the World Series ends to negotiate a deal with Cespedes. After those five days, they have to outright release him and they cannot extend a qualifying offer (so no draft pick) and teams can't re-sign a player they released until May 15th.
Basically, there's no way in hell he's staying with the Mets unless they offer him something outrageous in those five days like 5/$125 million (and their owners are cheap as hell). No agent is going to let a player coming off the best season of his career (at least, up to this point) not hit the market and get the best offer available.
Detroit has a much better chance of signing him now than had they kept him.
Ken Rosenthal broke it down:
Tigers might trade Cespedes ... to keep him?