He was one of our best when he came on, but I think people went OTT with "most important, our best/ most creative player" thing. There are other times when our supposed more talented younger players on their top gear look better.
What Mata offered was experience through his football intelligence. For our goal, it's not the first time in game, one of our players were put into good position but failed the delivery, not able to earn a corner. I can remember Rashford at least on 2 occasions instead of first time cross, just try to take on defender then change his mind at last minute and botch the momentum of the play. Lingard, Alexis, Young are similar. Frustrating bunch. Took a touch or two too many. Even when they have time measure a cross, pass still execute poorly. Sometimes just goes with the flow put it first time into the dangerous area would help.
Anyways, our problem is nothing new. We need creative player. We have too many players we need to feed but now down to only Mata with Pogba's poor form. Alexis possessed more creativity (more variety in his play) than Martial as for the experience. However, like I said when he was still Arsenal player, he is still a player feed on supply than the creator, and he has not been hitting top form this season even for Arsenal let alone us when joining in January.
All in all, Mata must play for me till the end of the season though. It's more balanced that way.