#2 Won't be an upgrade to the current crop
The Brazilian works best when deployed in the left-forward role, while he is also effective as a central attacking midfielder. While playing in the forward role, Coutinho likes to cut inside from the right and it is from this position that he is most effective. The Brazilian has a beautiful right foot and his ability to score from distance is extremely good. The problem is, United already have Anthony Martial playing in that position, and the Frenchman is, on current form, better suited for the role than Coutinho.
The ex-Liverpool player could also do a job as a central midfielder, where his passing abilities and vision would be an asset. But he would have to displace Paul Pogba for the role and the Frenchman is arguably one of the best players in his position in Europe, and certainly better than Coutinho right now.
A midfield of Coutinho, Pogba and Matic would lack bite. Pogba would also have to play out of position, something that is unimaginable at the moment.