To be fair to United, they did play a lot of reserves and bench warmers.
Welbeck, Giggs, Chicharito, Lindegaard and Anderson are senior players that don't get that many starts and can be classified as bench warmers. Then you have players like Kean, Buttner, Wooton and Powell who are more or less reserve team players. Rafael is a definite starter, while Nani starts every other game.
You can't compare that to Chelsea who started the game with regular first teamers such as Cech, Cahill, Luiz, Mikel and Mata. Then they brought in others like Oscar, Hazard and Ramires. You also have players like Sturridge and that speedy leftback who are senior team players that get quite a few minutes.
United did well taking into consideration the differences in the squad played in the game.