United had been under funded on the transfer front for years. It was like watching a car age. You keep patching things up and spending what you need to keep the car running but eventually you need to buy a new one. United squad was old, lacking real quality in most of the team and full of players who were about to hang up their boots or were in decline. Fergie wasn't going to rebuild a United team in the last season or two he was there, he should have been maintaining the level of the team from when we had the likes of Tevez, Ronaldo and Rooney leading our line. Ferguson watched the quality of player decline slowly until he left.
I would put good money on the fact that City will win the league this season and then spend a lot of money on top players in the summer despite being far stronger than everyone else in the league. When you are at the top you should consolidate your strength, not watch it ebb away as other teams slowly catch up with you.
Which has what to do with what I was talking about?
New manager comes in with previous manager having spent 100mil in the previous 5 transfer windows, having a huge amortisation cost going through every season, with 10 players on newer 150k a week 4 year contracts such that pushing any of them out is expensive and difficult. The new manager has to use the players fergie picked and can only make minor changes. Due to less spending, no new star players on huge contracts the new manager was free to spend more on the players they really wanted and make far more changes without being restricted. Fergie left a team ripe for rebuilding that was easy to change drastically even in one transfer window. A team isn't great for rebuilding if you can only afford to buy 1-2 players and everyone in the team is on new expensive contracts.
Utd were in the best position they really could be for a new manager to build a new squad very quickly.
As for City, City already massively backed off their spending this year. Two years replacing an ageing defence (particularly at fullback) and they now need fairly minor maintenance in terms of player turn over to remain fresh and competitive.