Disagree that it's a tough loss, he was gone after next year anyway and they likely weren't contending next year anyway, so it gives them an opening to audition guys the rest of this season and next, like Chad Green last night for instance (that was Eovaldi's spot). It might even free up another 40 man spot this winter if they decide to non-tender him as seems likely.
So the only loss is in potential trade value, and they're pretty loaded with prospects already. I'd be much more upset if it was Kaprielian or Sheffield or Acevedo or Severino or Green or Cessa or Adams, those are the guys who they hope to step up into the rotation by the start of 2018. Montgomery, Enns, the list goes on. They need to give internal options more chances.
Edit: And of course it sucks massively for him, and I do feel bad for him, I always liked him and hoped he could harness his potential for NY. He never really did, though.