It's a combination of injuries (RVP, Rooney, Vidic and WGT have all been out a lot this season), age (Giggs, WGT, Potty Paddy and Scholes), poor signings for the last few years (De Gea and RVP excluded), but Kigawa, Young, Zaha, Anderson, Valencia represent £80 million of moderate ability and...
United had a wage bill of £162m in 2011/12 and this increased 11.6% in 2012/13, which seems inconsistent with your view that they're struggling.
Chelsea and City do have higher wage bills, a lot of Chelsea's in due to management payoffs.