This has been discussed before in this thread. Mata requires much tweak to the system. Mata is not a driving force in the team (He is more of final ball specialist and finisher), so not someone you look for inspiration in case the team doesn't click. He is not fast to be fox in the box type nor physical strong to use as a wall. Mata is defensive weak (not that he doesn't work hard), so there is better player if you need to protect the lead. With all that in mind, Matat is more a rotation option, which is better to go to younger players and lower wage.
For people who keep arguing for Mata about #10: Mourinho #10 is not he traditional type. Mourinho asked more from his #10 (and other positions too) than just the generalized cliche of work hard, track back... Mourinho tactic/ formation changes throughout the phase of the game, throughout the area of play than a rigid uniform 4231 433. Watch the game above and the highlight tactical use of Di Maria. Di Maria is an right winger in attack, but was required to be right wing back in phases where Barcelona pinned Real Madrid down. Ironically Di Maria played huge role in the goal attacking from the left side!
Ozil pressed Busquet if the play is near the midline, but required to do a defensive job pretty much as traditional holding midfield if Real Madrid being pinned back. It's not simple dropping deep for Ozil. He needed to show intensity and tactically defensive quality, as Barcelona means business in the area Ozil watched Over. Someone as defensive nous and high work rate like Park being used primely to plug to midfield hole was got overrun in that CL final, I have my doubt with Mata defending this area against Xavi, Messi Iniesta,... who all love to drift into this area.
Use LVG excuse as much as one can, LVG played Mata a lot and his bad management while hindered Mata's ability some ways, his possession football still favored Mata than Mourinho's. Mourinho likely plays counter attack, and rock solid defense shape is the corner stone of his philosophy. Mourinho already made statement in his press conference how he wants to play: vertical pass. You don't vertical pass when the opponent camp in the box your players and there is no space behind.