This is just wrong. Regardless of how wide you're right winger is, if the ball is on the attacking sides left wing, your left back will ALWAYS be tucked in... regardless of who is out there. Defenders should really be no more then around 10 yards apart, at all times... this is just basic defending, it's got nothing to do with Mata, or Memphis.
It doesn't make Memphis' job any harder, at all, he still has one full back to deal with. It's not like he has an extra defender coming across to him or something. Teams when they play against United will always keep their defence and midfield compact, regardless of how wide your wingers are. It literally wouldn't make any difference. The only thing you could say is that Mata drifting inside brings his full back with him, thus creating less space for Rooney/Martial - but a) this has no effect on Memphis and b) it just leaves more room for your full back to penetrate, which should - in theory - be a good thing. Also, if Mata is drifting inside to create and score goals, then that is literally the best way to break down teams who are trying to be compact and tight.