A hat-trick by Wayne Rooney, a Ashley Young double and goals by Danny Welbeck, Ashley Young, Park Ji-sung and Nani inflicted an embarrassing 8-2 defeat on the Gunners, who scored through Theo Walcott and Robin van Persie.
Speaking after the match, Ferguson admitted: "It is (a surprise) because when you play Arsenal you expect a difficult game.
"If you look at Arsenal the team is weakened - but we still did the job.
"We got careless at times and they made chances because they still have quality up front
"But overall we are very satisfied that we kept the performance levels up."
With his counterpart Wenger likely to come in for more criticism this week, Ferguson again leapt to the Frenchman's defence.
"I think it is unfair to criticise him," he said.
"The job he's done for Arsenal and the philosophy he has - he has given Arsenal some very entertaining players.
"He's also sold well and looked after the Arsenal coffers. People forget these things. It's a cynical world now and when you lose a few games the judges come out."