Chelsea fan here. Prime Matic was in 2014/2015 in the 4231 alongside Cesc. That season he was a true destroyer, and won so many tackles like Kante does for us now. Incredible player that season.
After the Barnes horror tackle he stopped going in on challenges. This past season although he was in the midfield 2 he rarely made tackles. He was more about enforcement and containment. Kante won most of the tackles.
Matic's playmaking is underrated, as is his passing. He's not a creative type but rarely misses passes, and if a player is open he will pick them out. He also can make great runs into the box, and is very underrrated with his dribbling. He will also very rarely lose the ball and knows how to shield it really really well with his huge body. He won't however create something out of nothing like fabregas can.
Matic's role for us is very much now a ball carrier B2B rather than a true defensive midfield destroyer. In my opinion He's unlikely to ever return to that same level in 2014/2015 where he won so many tackles and could cover for cesc (and if he moves to united pogba), so tbh I'm surprised mourinho is after him. But if mourinho can make him even half of what he was in 2014/2015 he could definitely help united, but his play style now is just so different even though it's hard to notice without watching him regularly.
From Chelsea's perspective it wouldn't make that much sense to sell him, because he and bakayoko would be great for rotation. But the quoted 40m is definitely a much much better deal for Chelsea than united in terms of value and price.