United quickly forged ahead through Dimitar Berbatov's header against the Black Cats and, after the Bulgarian had doubled his tally after the break, the game's pace dipped markedly for the final half-hour. With Birmingham's trip to Everton waylaid by frozen pipes at Goodison Park, Sir Alex admits he will have to factor in the midlanders' added freshness.
"It was a comfortable performance," the manager told MUTV. "I think in the first half we were absolutely fantastic; it could have been any score. Second half I thought we maybe put our tools away a bit and saved our legs. We knew the Birmingham game was off and they were very economical in the second half, put it that way.
"You have to. I took Anderson and Ryan off. We've got a game in two days' time of course, and with Birmingham not having played they'll be nice and fresh."
United's searing opening to the victory over Steve Bruce's side served up some of the most eye-catching football of the Reds' season to date. Coupled with another imperious defensive display, Sir Alex feels his side are coming to the boil at an ideal time.
"We're coming to good form," he said. "In the last few weeks we've played some very good stuff and they're getting that consistency - particularly defending. We're really looking solid at the back and these are good signs for us, and it's an important time to get that."