United, who have drawn their opening games of the last two seasons at home, were less than convincing against newly-promoted Birmingham City.
Although the Red Devils may have lacked the sparkle of Ronaldo, who joined Real Madrid for £80million in the summer they did have Wayne Rooney in inspired form.
The England striker scored the only goal of the game and could easily have had a hat-trick.
Rooney started and finished the move which brought United all three points.
Finding Dimitar Berbatov midway in the Birmingham half, Rooney kept on running as the Bulgarian headed the ball to Nani on the left.
The Portuguese winger produced a precision cross into the Birmingham box where Rooney rose above Franck Queudrue to produce a glancing header from eight yards.
The ball hit the foot of the right post and rebounded to Rooney who prodded it home from close range.
Rooney could have had a second goal seconds after the interval when he hit a spectacular swerving 35-yard shot from the left which was brilliantly turned behind by goalkeeper Joe Hart, who is on loan at St Andrew's from Manchester City.
Rooney was a whisker away from another goal six minutes from the end when his header from Ryan Giggs' left-wing cross went inches wide of the post.
Between the Rooney efforts, Berbatov saw his 71st minute header from a Giggs corner cleared off the line by Lee Carsley, while substitute Michael Owen twice came close to scoring in the dying minutes.
Owen was marginally offside after running on to Luis Antonio Valencia's through ball before rounding Hart and netting, while in injury time Rooney's superb chest down put him through, only to be foiled by the Birmingham goalkeeper's left leg.
Birmingham had two golden chances. Queudrue saw his 37th minute header from Sebastian Larsson's corner headed off the line by Patrice Evra.
Then, in the 78th minute, substitute Christian Benitez was denied by a magnificent save from United goalkeeper Ben Foster, who hurled himself across goal to turn away a close-range effort.