Gundogan's injury is categorically not a one off thing. It's chronic, but to what extent is uncertain. It could just be a case of extra physio sessions after a game, or it could end his entire career. In other words, United would be gambling big.
Carvalho's too raw to start for United. If I was United manager I'd sign Milan Badelj.
The dip in Kovacic's performance happened BEFORE he was dropped, not after. It was the cause of him not getting consistent games, not the result of. Your third paragraph really shows exactly why United don't need him; he's not a holding midfielder, and he needs one beside him to perform well. United only have Carrick as a proper holding midfielder and he's knocking on to the point where he won't be able to play every game. United have plenty of creative talent in Rooney, Mata and Januzaj, so why go for the less defensive and more creative option? It's pretty senseless. Of the three you mentioned, Carvalho's easily the safest bet, but he's going to go for a LOT. Mike. pointed out to me that his release clause is around ?37 million, which is a truly ludicrous amount for someone of his current ability.
Like I said. Milan ******* Badelj. Someone please sign him soon, else I'm going to go mad. I spent ages extolling the virtues of Nemanja Matic and then Chelsea buy him and he's one of their most important players I should be a manager grumble grumble