Thing is, I still think we're going to get a top four place with the squad we have. I also think we're not going to retain the title, regardless of who we sign in January. So for me there isn't much to gain or lose with a short term signing.
I agree that we also need very good/solid players in our squad. We currently have an abundance of them though. Carrick, Cleverley, Fellaini and Fletcher all fall somewhere between very good and solid. What we're currently lacking is top quality. So another very good/solid signing won't do much for us.
Midfield is the one position where we don't have a player who at least has the potential to be top class. That's the type of player we need to improve, not yet another squad player.
Moyes had it right earlier in the season when he said we need a few players who'll improve our first team, not improve our squad. By "improve our first team" I'm thinking they at least have to be around Carrick's level.