My assessment is that Morrissey lost his game, maybe through injury, maybe by losing a partner with chemistry, maybe with added responsibility, or some combination. But he just isn't playing with the assuredness of a few seasons ago, and he's making more errors.
I still believe he has the skillset and smarts to be an effective top 4 D, but he needs to find the game he had. He plays well for stretches, and then has stretches where he seems lost, fumbles the puck, etc. I think he is now lacking confidence and playing tentatively too often. I'd try him for an extended time with a stable partner like DeMelo, give him a bit less ice time, and just tell him to play a fast, aggressive game.