Edit: Sorry for the boring blocks of text but I wrote this in a hurry so please bear with me and try to read through the whole thing if you can.

The Fellaini problem

With Fellaini in the team, and without Mata, our midfield is far too easy to counter and even our attack is too easy to counter. Fellaini is a player with limited technical ability and a very poor passing range. Here is why Fellaini and Pogba simply doesn't work.

The areas where Pogba receives the ball is very deep compared to what area of the pitch he received it at Juventus. He is often dropping deep to collect the ball because Fellaini doesn't pass it forward to him. Fellaini rarely passes the ball forward, if ever and doesn't have the vision to conduct play himself from a central position (a la Scholes/Alonso/Schweinsteiger).

Having dropped deep to get the ball from Fellaini, Pogba then attempts to either spread the ball wide or dribble it forward himself because there is nobody to link attack and midfield. At this point he is dispossessed or intercepted because no human can be expected to beat multiple players without fail on every occasion. There is no supporting player in midfield. He is the sole chance creator and conductor from the centre and when teams have a packed midfield, it is all too easy for two midfielders to rush him and force the ball to go wide, which is so often the case. At this point, defenders can double mark Zlatan while the fullback and wide midfielder stops our winger from crossing, thus effectively neutering our attack.

Seriously go back and watch matches where Fellaini and Pogba are playing together and that is always the problem. It is literally stop Pogba and you stop United from having any threat down the middle. The ball is always forced wide and believe it or not, we don't have decent crossers of the ball. Valencia is very hit or miss. Lingard is poor with crosses and Rashford/Martial drift inside. Where does that leave Zlatan? Isolated and with very few chances because either he has to drop deep to receive the ball or wait for a cross where finally if 2 out of 5 make it to him, he is double teamed and the ball is cleared. Our team is not good at getting crosses in and even when they do, there is mostly only Zlatan in the box. One player in the box will not score you goals off of crosses, especially when that seems to be our only method of chance creation so far. Zlatan's chance against Liverpool? cross. Chance against Chelsea? cross. Both were our best chances of the game.

Pogba needs to be getting the ball a lot forward on the pitch, the areas between the midfield and defence, where midfielders can be deadly. That was the area De Bruyne was constantly getting the ball when we played City and why he was so effective. That little bit of space gives you extra time to spot a pass or go for goal.

No attacking identity

This is where Mourinho's fault lies. I, for one, haven't seen a sign of how Mourinho wants this team to attack, what channels he wants them to use while attacking. If he wants us to be a crossing team, (which the presence of Zlatan suggests), there should be a lot more crosses than we have been seeing and switching play should be faster. More players should be flooding the box during crosses. This is why Fergie's United were so successful off of crosses. We had Scholes, receiving the ball from Evra and immediately switching the ball to the other wing, where Nani, Valencia whoever looked for a cross where there were at least two strikers in the box, with a midfielder standing on the edge and the other winger on the back post. Our main method of chance creation is crosses, yet we don't have enough players in the box, not enough decent crosses, or fast switching of play to score off of them.

If we are not a crossing team, then what is our playstyle? How are we supposed to be creating chances? Either way, Mata must play.

Why Mata is (part of) the solution

Now, with Mata in the team, Pogba is no longer the solitary chance creator from the centre. We have two players in the middle who can drift into dangerous areas. Furthermore, Pogba actually has the vision to pass the ball forward to Mata where Mata can then link up with either wide player or conduct play as he so effectively does. This gives us more threat down the middle and forcing the ball to go wide and then double marking Zlatan will no longer be the way to stop us from scoring. Mata can shoot from distance or find a pass and so can Pogba and we have Herrera holding play at the back.

If we want to play 4-2-3-1 Pogba, Herrera, Mata is the only way to go. A Mkhitaryan out wide would be even more dangerous but we don't know how he will play because he has rarely played this season.

TLDR; Mata needs to be played and a Fellaini-Pogba midfield doesn't work. However, our attack is still too predictable and lacking in identity and this is what the manager needs to fix.

On an unrelated note, Pogba has looked the worst after our Europa League games (v Watford and v Chelsea). Maybe Jose is playing him too much. He played almost 270 minutes in 7 days and looked knackered in both games.