Why can't Mata will start when he's been starting our games recently? Personally I don't fancy Mata but I'm not the manager so there is a debate that Jose might want to start him and keep the same XI as our previous league game and bench Sanchez since he might need a bit more time until he is gel with the system and new team mate.
I don't see why playing 3 at the back can only be used if our opposition is playing 3 at the back. May be Jose wants to be more solid in defense by adding more defender. Well, technically it's 5 at the back and we are playing away so I won't be surprised if we go 5 at the back.
Mata has never started any big game for us this season, whatever the injured/suspended players were. He's too slow and weak where we have much more fresh, pacey and stronger options like Sanchez and Lingard.
Mourinho has only played 3-5-2 when the opponent was playing with 3-4-3 or 3-5-2.