Although a United fan, I'm one of the biggest complainers about our performance levels and squad but I certainly don't see our current situation as a crisis...
Goalkeeper we're set for years. I think we have a very good selection of defenders. We have the Vidic/Rio replacements already at the club and the former's performance levels are still very high anyway. I am however increasingly annoyed about the amount of players we have who have fitness issues and having to constantly rotate and rest certain players because of medical reasons is a big strain on the team, and this does apply to our centre backs. Rafael is considered one of our top 3 performers this season and is ever improving, brother Fabio returns in the summer from loan spell (what a waste of time that was). Evra was on the decline for 2 years but picked it up a bit this year, but it wouldn't sadden me to see him be replaced, he's complacent.
We have an excellent set of strikers, no idea why we'd mess with things by getting rid of Rooney/bringing in Falcao/Lewandowski, it's just Fergie's solution to everything seems to be to sign a new striker.
Wingers went from being one of our strongest areas to our weakest in the space of a year. They have been terrible season, we've ruined what was a very good player in Nani who won player's player of the season in 2011, he seems destined to leave now for peanuts probably. Zaha has been bought, we'll need a top talent to replace Nani when he leaves and hope Valencia finds the form that wont him poty last season and Young stops being quite so useless.
The midfield issue is a joke. We've needed to sign one for about 4 years but haven't even come close. Carrick is our only defensively capable player in the squad, if he got injured we'd be absolutely screwed, it's such poor squad building. He's having a top season though and I don't think listing him in the 'old and a liability' pile is very fair at all. Our inability to let go off Giggs and Scholes is another cause of frustration, they're a huge part of the reason Pogba left and why we put off buying a midfielder year after year, they have to be constantly rested and rotated, they really aren't top players any more on an even slightly consistent basis. Ferguson also has an obsession with Jones in midfield when he's clearly a defender. Cleverley's a pretty decent player, and Anderson's a decent squad option too but he barely gets given a game because Ferguson would rather give 40 year olds and 20 year old centre backs game time in midfield instead - he will most likely be sold as well. Ideally we'd sign 2 good central midfielders, one more defensive and one more attacking, cut our reliance on the oldies and have a strong quartet of Carrick, Cleverley and 2 new faces to play around with, but it wouldn't surprise me if he doesn't address the problem for the 9th transfer window in a row.
I think Ferguson gets more praise outside of United than from our own fans nowadays to be honest. I often read on here and other places how "you have to admire Ferguson, getting such an average squad to consistently win trophies", or "Ferguson's team mentality to win games when playing so poorly has to be admired"... Well, surely he has to take responsibility for building a squad with significant weaknesses despite splashing a lot of cash over the past 2 summers and also some responsiblity for his team so rarely putting in a good 90 minute performance. I could count on one hand the amount of top team performances we've put in this season, and we've played way over 50 games. We should be going into the season thinking we're one of the strong favourites for Europe's top prize, and with such a weak midfield how can that ever be the case? I'm not even sure he sees any issues there, some of his comments are beyond deluded like how he thought we'd have got to the final last season if we'd gave got past the CL group stages, and his standards for what he considers a good performance from us is embarrassingly low, the West Ham game a few days ago, our display was described as "magnificent" and the loss to City was an "excellent performance".
I think this is Ferguson's big worry and maybe the reason he's continued so long. I don't think there are any real candidates in the EPL, maybe Villas-Boas in a few years. However if he was to leave in a season or two, they'd really be struggling for a manager unless the went for a Mourinho, Klopp or Solskjaer etc. The only other option is to appoint someone from within, a Giggs maybe.
It depends on the timing, on who is available when he decides to call it a day. I imagine we'd look straight to Mourinho/Guardiola but I doubt whether we'd buy them out of their contracts at Chelsea/Bayern, or if the timing is right for them to take their chances at Old Trafford. Outside that I'd prefer to see the club take a look at Klopp, Van Gaal, Hiddink, Heynckes, Pelligrini etc. before we resort to the likes of David Moyes. I don't think there's a shortage of candidates at all to be honest.