Mata wont make a difference, just like Ozil hasn't made that much difference for Arsenal IMO. Arsenal's biggest plus this season is their two CBs having their best season ever, Ramsey turning into a quality player and Flamini coming back to hold the middle. They are the core of the midfield and the defence.
Utd have RVP and Rooney along with Kagawa, Januzaj and Valencia they dont need another attacker its unfortunate that they are injured but that isn't the problem. They have two CBs that are too old now and two others who are piss poor in Evans and Smalling, CM they have nothing, no strength and just Carrick who is also getting on. They have no core of the team left, Fellani was a very poor and rushed signing. Just look at Man City and Chelsea. Kompany, Yaya Toure, Aguero and Terry, Ramires & Hazard/Oscar, core of a solid team.
Could also compare them to us. Utd's best players now are RVP, Rooney and De Gea, it's a bit like for us when Vlaar/Delph are out we are so much worse off its unbelievable because there's nothing between Guzan and Benteke.