Some thoughts about Morrissey...
He had a very good track record as a junior and AHL prospect. So his good play earlier in his career wasn't all that unexpected. He has a strong talent base and can process the game quickly.
In the past, his main deficit has been when he's played tentatively, and that's returned into his game.
His performance has clearly fallen off since a couple of key events: the departure of Trouba (and Buff), 2) a substantial injury (probably a fractured or "separated" clavicle).
I think his poor performance since is perhaps related to both changes. As
@garret9 indicated, Morrissey might need a different type of partner to be fully effective - perhaps one that can carry more of a load in puck carrying and physical defensive play. I also think Morrissey needs to regain his aggressiveness in defensive play, particularly in his end of the ice. He's letting the play come to him more than he used to.
I'm not sure whether the Jets have an ideal partner in their system, or on the trade market. DeMelo might be a good complement defensively, but isn't all that physical or good at carrying the puck. Heinola is a great puck mover, but lacks defensive physicality. Samberg is good defensively, but not as good moving the puck. Maybe the Jets thought Poolman could be a poor man's Trouba, but he has a lot of inconsistency with the puck.
Trade targets? Savard is a big physical D, but will Morrissey be able to provide more of the puck carrying / movement? Ekholm might be ideal, but would force one of them to their off side. Ristolainen? He might be like a more expensive Poolman in terms of style and defense, though better offensively.
Would the Jets consider trading Morrissey? I highly doubt it. He's too well respected by the coaches and brass. But if they did, replacing Morrissey with a D like Lindholm or Ekholm might improve the Jets.
