Last season we had too many midfielders, hence why we sold Schnerderlin (who barely got a game) and Bastian (who trained with the reserves for a fair bit). If the Matic deal goes through and assuming we're playing a midfield 3, we'd have Carrick and Matic to play CDM, Pogba and Herrera taking the other 2 spots, with Fellaini, Pereira (came back from loan so he's like a new signing in a sense) and potentially Fosu Mensah as the bench players. That's plenty of options. It would make little sense for us to sign Dendoncker now and sell Fellaini, especially if Dendoncker is brought in to be bench player as I don't think he'd be particularly happy to come without guaranteed game time. Fellaini is also happy to play from the bench and having him around gives us squad depth.
Dendoncker could well sign for United, but I reckon if it were to happen it'll be next summer, when Carrick retires and Matic has settled in.