Rio, Vidic, Carrick, Evra were coming to the end of their careers, and we still depended heavily on Scholes and Giggs who retired. Rooney had also been fading at this point in my opinion. RvP was great in his first season and carried an otherwise lacklustre United team to the title, and was understandably irked by Fergie retiring having made the move to us for silverware.
Rafael, Kagawa, Jones, Evans were decent players, but not great. Rafael had his moments, but these were squad/utility players at best. Kagawa was a good player but he had stuttered at United and his move hadn’t worked out so far. Smalling and Jones were similarly not great players either. There was potential, but we know how that worked out for the pair of them.
When you say we had a great mix of experience and youth, we actually had a mix of formerly world class players coming to the tail end of their careers and youth players who hadn’t (and subsequently never did) make the step up, with a lot of average to decent players in between. It’s easy to just dismiss all of these things by virtue of us winning the league that year, but the squad had been fading for several years by that point. It was also Fergies team, and I personally don’t believe any other manager could have got as much as he did out of that squad. But again, we made some calamitous decisions after his retirement which made a tricky situation much worse.
It’s quite a bizarre statement to say that Mancini won the league without signing good players. I don’t even know where to start there.