I'm not a fan of him either, but I don't buy this at all. He was poor in 15/16 like all Chelsea players. In 16/17 he was actually quite good and one of Chelsea's most influential players on average (only Fabregas and Azpi had more passes per 90 min). That despite playing in a position and system that did not suit him particulary well.
In my opinion Matic is a type of player that provides perfect cover for the central defenders. He is great when he gets to sit deep and use his positional sense. Either in a 4231 or as the midfielder that provides balance in a 433. In Chelsea last year Luiz and Kante made him less valuable. His biggest strenghts was excessive to the team. Furthermore, as one of two midfielders in a 343 he had to move up and down the pitch a lot. For a rather big and heavy player that is not ideal. I would not be surprised if all this was part of the reason he wants to move.
And although big and heavy Nemanja Matic looks very fit! He is of similar build as Zlatan, and appears to be a model professional. Michael Carrick, a similar player, was at his peak in 2012/2013. In my opinion, the same was the case for Xabi Alonso, a similar player. Both of them was in 12/13 three years older than Matic at this time.
All in all, a boring player, way to expensive. But I actually believe the best is yet to come from him. And I also believe the qualities he would bring to the table would be perfect for us.