Matic was one of my favourite players when he returned; I've always had an affinity for defensive midfielders. Here's some observations of mine from watching him very closely over the past few years.
1. When he first came back from Portugal, he was capable of single-handedly controlling a midfield by himself. Here's a video-I think this was his second or third game for us, when City were the best team in the country and Yaya was the best midfielder in the country. It remains one of the better performances I've seen from a player in a Chelsea shirt, considering the circumstances:
The following year, he could still largely do this (though Fabregas actually was more defensively useful than he was given credit for), but a lot of this changed after an absolutely shocking challenge from Ashley fecking Barnes.
2. Ever since then, Matic's defensive numbers have been in steep decline, in particular his tackling-he went from attempting 5.7 tackles per game to 3.6 last year to 2.3 this year. This hasn't been because he's been getting more interceptions, those have been at 2.0, 2.0, and 1.4 over the past 3 years. WARNING! CONJECTURE ALERT! It seems to me that he's become very hesitant to extend his legs into challenges, thanks to (again) Ashley fecking Barnes:
With the obviously massive caveat that this is Squawka, you can see the massive dip in form that this caused, he really fell off a cliff for a while before recovering somewhat towards the tail end of the season. I can tell you that at least by the eye test, he has never been as aggressive as he was for the first 12 months he was back in England.
3. In terms of his passing, he is only comfortable making incisive passes with his left foot, which can be a problem at times. He is pretty clumsy on the ball and can be ponderous/unimaginative, especially when closed down. When in a 2 with Fabregas, Cesc would draw enough attention that Matic would often be in space, which would be ideal for him-I think Matic struggled at times because opposing teams were more worried about his passing than Kante's this year. For me he fails to shift the ball quickly enough for two reasons, first because he is only comfortable doing so with his left foot and second because he is very tall and needs to adjust his lanky frame around the ball to position his left foot to play it. There was a noticeable difference in the speed with which the ball moved from flank to flank between wingbacks when it went via Matic versus Kante this year, for instance, and these things can be important.
Another worrying trend though is that his dribbles are way down as well-from 2.5 total/1.9 successful in 14/15, 1.6 total/1.2 successful in 15/16, to 1.3 total/0.8 successful last year. Obviously not a priority for a holding midfielder, but his ability to dribble past and break pressing was vital to help make up for his somewhat imprecise passing.
Look, I'm really not trying to be a WUM or anything, I just can't for the life of me understand why United would spend £40m on a soon to be 29 year old for whom every statistical profile plus scouting report I can find indicate he's in decline. If you were getting the Matic of 3 years ago who destroyed Yaya Toure at the Etihad then of course yes, you do that deal in a heartbeat, but that player has been gone since February 2015.