Ferguson said he was going to make seven changes from the side which beat Fulham last weekend, but Rio Ferdinand and Michael Carrick were the only regular starters omitted from the line-up for the Old Trafford tie.
The move paid off as United ran out 2-0 winners thanks to goals from Ryan Giggs and Robin van Persie to move 12 points clear of Roberto Mancini's men, who were beaten 3-1 at St Mary's.
And it puts them 12 points ahead of City – as bookies Paddy Power yesterday paid out on a United title win.
Asked if City's defeat 24 hours earlier had changed his selection plans, Ferguson admitted: "Yes, it did.
"I thought I'd make about seven changes, but when I saw the result on Saturday I knew how important the game today would be. We can make more changes later in the season."
"We have to win our games anyway, it doesn't matter what happened with City. That's the way we should look at it."
Ferguson was full of praise for evergreen Giggs, whose strike ensured he maintained his own record of scoring in all 23 Premier League seasons.
"His legacy lives on," Ferguson said. "He's up and down that pitch all day and has shown fantastic energy for the game."
The player himself was more concerned with getting the three points than personal milestones.
He said: "It was an important goal. It is important to continue our good form and get the results We have been in good form."