Ok, if Moyes was going to stick with Fellaini and Gibson infront of the back four how many games are they likely to miss over the season? Felli missed 4 league games last year and Gibson was a virtual ever-present since he signed. Would Moyes drop one of them to accommodate Essien? I can't see it. So the Chelsea player would only get a handful of games when one of them wasn't available. Even if one of their knees did spontaneously combust we have the players in the squad to do an adequate job in the interim. I would be really surprised if Moyes made it a priority to find back up for that area of the pitch.
As far as I'm concerned Fellaini will play up with Jelavic much more often and that has opened up a space next to Gibson. While Heitinga, Osman or Neville could step in to the breach a specialist would be the preference. I thought Junior might have made the step up from what I saw in pre-season but I suspect his size might have been an issue.
Why would we stop buying anyone once we had eleven players? Moyes' job is to keep finding people who are better than what we've already got. I'm sure Vadis won't start every game. It's usually his way to ease signings in over a period of about 6 weeks. I'd imagine that he will get his fair share of games though.
In answer to the part of your post I've quoted in bold: it's called "rotation". Teams don't win anything, squads do. Squads involve more than eleven players, and with careful planning you assemble a squad so that you have more than eleven decent players. Irrespective of how many games Felli and Gibbo did or didn't miss last year, I see no-one else in the squad providing competition or cover in that position. I would rather have the three of them each playing 30 games than relying on two of them to play 45 each. And that's the case whether
Moyes managed to get Essien in or not. Or if Ofoe joins us. Or if we get Hargreaves or bloody Ballack in either.
Now to the italicised part. I am happy to see Felli pushed forward on occasion when the tactics are such that he will be beneficial there. Against United they had no-one who would dominate the midfield (Scholes and Kagawa both very technically gifted players, but not dominant ones) and only one functioning centre-half. With neither Mirallas or Naismith being ready and Felli having a massive aerial advantage over whoever Fergie put in as Vidic's emergency partner, it made sense. The only other alternative being a straight 442 with Vellios or Cheebs joining Jelavic. Osman, Heitinga and Neville cannot perform in the CDM role to a standard acceptable in premiership games. You may well be right about Junior - hopefully he will grow intot he role as he matures.
Finally the underlined portion. I'm not suggesting
Moyes SHOULD stop buying players once he has eleven. I was highlighting the fact that under your thought process, that's what would happen because as you said:
"If we had got Essien and paid him fortunes per week do you think that he'd be on the bench? And twin destroyers need to play together regularly to get in sync so I can't see rotation being much of an option." - the clear inference being that Essien would be an immediate starter because he would be a new purchase/loan, and you don't believe in rotation.
It's not Moyes' job to just keep buying better players, sometimes he needs to buy backups / cover / rotation players to fill out the squad - Oviedo and Vellios being obvious examples.