To my eye, the solution is really simple: bring in Ronaldo.
If we can assume we'll have a healthy Vidic for most of the season, we're going to keep a shitload of clean sheets. That's just the way it is. So I'm not into going after a shit-kicking midfielder right now, although if Bayern put Schweinsteiger up for sale (which they never well), I'd open up the checkbook for him immediately. And on those days when we do need a negative specialist to grind down an attacking threat, I'm happy to throw Jones into the lion's den. Bizarre facial expressions aside, he's actually done a pretty job in midfield.
Do this, as costly as it will be, and we'll haul in the prem trophy next season in a cakewalk and make it to the semifinal of the CL:
Carrick
Valencia/Kagawa/Ronaldo
Rooney/RvP
This assumes Nani has left us. If he stays, he and Valencia can compete for the right mid spot. Cleverley is more than an able understudy for Kagawa. Ronald is pretty decent. Hernandez and Welbeck ain't so bad. Young is a decent squad man. Depth is not our problem...our problem is the lack of a reliable executioner beside RvP. That reliable executioner needs to be Ronaldo.
The objection to this, one supposes, is that we might be a little soft in the middle of the pitch with Carrick and Kagawa and that would be a reasonable concern. But I'm going to go back to my assessment that Vidic is a monster. De Gea is on the verge of greatness. And I'm going to stand by my assertion previously that Valencia does a very good job defensively.
If Moyes thinks it can work, perhaps have Roo sit back as a true central mid and push Kagawa up. That would solve the "softness" concern, but I just don't know if Roo has the fitness level to pull this off. Maybe it doesn't matter as we wouldn't need Roo to run box to box against most clubs and he can save himself for the big matches.
So I just wouldn't bother with Fellaini. He deserves to be starting on a regular basis but I don't see him being the right fit for United unless he's happy to be a squad man. Strootman probably would be happy to be a squad man, so I wouldn't object to bringing him in to learn the trade in the English prem as Carrick's engine begins to slow down. But neither should be the immediate focus.