This is not about bashing Mata. I'm only gonna talk about how I've seen the player since he joined Chelsea and how I see him now(the positivity comes at the end so bare with me. And sorry for the long post.
I have never been a fan and haven't had much positive about him to say. I could see why Mourinho didn't want him back then. I believe he's too weak, easilly dissposessed, too slow, he just gets run over by attackers when attempting to track back and defend. It was Villas Boas who bought him in his first and only season at Chelsea. I think he only had one good season when he was basically their best player in 2013 I think. Obviously I wasn't watching every match so I can't say exactly how good he was, but I've seen enough to form an opinion. It's fair to say he wasn't my type of player. I clearly remember the match we played against Avb's Chelsea at the start of the season when we completely outplayed them. Avb's high line coupled with his new signing Mata and Sturridge on the wings who couldn't defend meant Nani(and others) had a field day. I remember when Mata tried stopping Nani but Nani just ran through him like he wasn't even there and went on to score a beauty.
After this game I thought to myself, the solid unpenetrable Chelsea is gone and I wasn't too happy about it, since it was always more interesting having someone put up a fight against us competing for the league. The reason I'm bringing this up is because Mata was part of that downfall for Chelsea(not a big part) but still replacing Malouda and Anelka who were strong experienced players for Mata and Sturridge was Avb setting his mark on how he wanted to play, which didn't really work out for him. Chelsea became soft and easy to score against. These were my first impressions of Mata, which I wanted to share. Although I had seen him for Valencia at times.
Having said all of this, lately I have started to appreciate him more, just like Mourinho has, I believe. He is an experienced player now and that is very valuable when most of our attackers are young, especially considering Zlatans absence. I think if surrounded by the right players and playing in the right system, he could be invaluable to the team. He rarely looses the ball(not because he's strong but he always realeses it at the right time, linking very well with others upfront, he can score, he can assist, he's creative and intelligent. He has also displayed more tenacity under Mourinho, you can see he works harder at chasing down more then I've ever seen before. I have actually seen him take the ball of players several times. He's also good at making runs in behind despite being slow, similar to Ibra and he knows exactly when to make these runs. He can't protect the FB though because of his physique which is a problem on the wing. If only he had some more strength and pace, he would be world class.
I still am not sure though if he's completely safe to stay here/play that much next season. He has been the best of a bad bunch and it could be that Mourinho only had to use him because of the lack of better options. When we get better players for those attacking positions, players of the Mourinho type, he could become dispensable. Especially if Rashford develops quickly and Miki starts producing, it will be even tougher for him. But we do have a lot of games in a season so I think he will still be useful to have.