The thing is that, playing Mata would be perfect if Valencia can provide a good/very good performance on the width. And I believe that was exactly Jose's game plan. But the fact is that Valencia underperforms, hence the "congested middle & without width" issue. Unless Valencia can deliver, playing a "wider" winger can balance out the lack of width issue.
Without Mata, Pogba and Mkhi can still provide enough creativity. And playing a "wider" RW give them an extra passing option, opening up more space and more link up play opportunities. All of these can further unleash their creative playmaking.
Also, it is not rocket science that breaking down a bus must be solved by "packing the middle with creative players". Using width is also another solution. Martial and Rashford have different attacking style. Rashford can attack the space whereas Martial can dribble and do some tricky move around the box. Both bring different dimension, more pace on width and more unpredictability as well.
All in all, unless our fullbacks can consistently give a good/very good performance, I think the "congested middle" is still a big issue this season, especially in breaking down a bus, as evidenced in the first 3 games. Counter attack is a different story, I think we should be good at that.
It is not so much about Mata is not good enough, is more about fitting the right player for the good of the whole system.