With 15 minutes left and United piling on the pressure at 2-1 down, Nemanja Vidic was shown a straight red for bringing down Steven Gerrard, and Fabio Aurelio curled home the killer third goal from the resultant free-kick.
"It was a nervy start from both teams but we took the lead and started to get a foot in the game," Ferdinand told MUTV. "Even when they were 2-1 up, we started to turn the screw in the second half and Carlos Tevez had a half-chance.
"But the sending-off and third goal came at a really critical time. I thought the referee's decision to send Vida off was debatable as I could have got over there to cover."
But Ferdinand refused to castigate his fellow centre-back, saying: "Vida has been one of our most consistent players this season and everyone is allowed to make a mistake, so we won't be getting on his back."
Ferdinand even managed to take a positive from what was a distinctly bad day at the office, adding: "Losing any game is disappointing, losing to your big rivals is even worse and the scoreline compounds that.
"But we have to turn this into a positive. This game will have torn out any sub-conscious complacency there may have been. We got a rude awakening today that if we don't go out and work hard as a collective unit then we won't get the results.
"We now have time to dust ourselves down and get our minds on the game against Fulham next week. It would have been nice if that game was right round the corner but we have to deal with what's in front of us."