The BJs never should have given up Syndegaard in that trade but D'Arnaud is another matter. The dude is constantly hurt and he is now out in California rehabbing with his own therapist because he, and a bunch of other Mets players, do not trust the Mets training/medical staff. D'Arnaud's constant injuries are a problem for the Mets, as is David Wright's spinal stenosis, which has sapped his power and also made him day to day for basically every game. The problem with having guys like this on your team, and the Yankees have had the same issues with Ellsbury and Teixiera and Gardner recently, is they just cannot be trusted to be able to play every day. Wright is a major problem because they have committed a roster spot to him, he is perpetually day to day due to the spinal stenosis, and basically the Mets have to find another third baseman who they can play every day if needed at a position that is very difficult to play, as Rob Refsnyder showed in spring training when he took ground balls to the face in two consecutive games, the second time having to leave because of the extent of the swelling of his face and cheek.
Don Mattingly's career went much like Wright's- a back injury derailed his power. Ironically he sustained that back injury while swinging for a major league record tying homerun in 8 consecutive games during the summer of 1987, a record later tied by Ken Griffey Jr. The Yankees were at least able to get some powerless, but reasonably regular production out of Mattingly. Due to the spasms that go with spinal stenosis, the Mets simply do not know whether Wright will be able to play on any given day and if so, what they will get out of him. He will probably be out of baseball sooner rather than later, which is too bad, as it was with Mattingly who was only about 32 or 33 when he was forced to retire, although he went out on a very high note with his memorable and productive playoff series vs. Seattle Mariners in 1995.