This United side has no fluidity without Mata. And that's Mata playing out of position on the RW. On that note, saying Mata can't play behind a striker is laughable. Last time he did that he won Chelsea's player of the season.
Gary Neville sums it up even better:
"It looks like you’ve got Rashford, Martial, Lukaku but they haven’t got that glue that just knits it together. Mata looks like he might become a very important player.
"Carrick, Pogba has sort of got the ability to connect back to front but Mata certainly has got the composure to hold the ball and allow those players to make their runs so they can get up the pitch and get into a point where they can play one-on-one.
"They need to be one-on-one. You see with Rashford [against Liverpool], Sanchez, Lukaku. Get them one-on-one around the edge of the box, they are probably as good as you are going to get.
"However to get the ball into those situations, as many times as possible with quality, you need someone to put the ball there.