Goals from Danny Welbeck (20), Nani (penalty 48) and substitute Darron Gibson (81) earned United a simple passage into the next round. United, who could afford to be without Cristiano Ronaldo and Carlos Tevez, were on top from the kick-off.
Their first opening arrived in the fifth minute courtesy of a fine through ball from Dimitar Berbatov.
The Bulgarian striker cleverly sent Ryan Giggs racing into the penalty area but the United winger’s effort was far too high to trouble Kelvin Davis in the Southampton goal.
In the 16th minute, Nani forced Davis into a fine save when the Southampton goalkeeper had to dive low to his right to deny the United striker.
Davis again kept Southampton on level terms two minutes later when he tipped a shot from Anderson over the crossbar.
But the goal United had been threatening finally arrived in the 20th minute through Welbeck.
Nani’s corner was headed against the crossbar by John O’Shea and Welbeck had the simple task of nodding home the rebound from point blank range.
Moments later a brilliant ball from O’Shea almost put Berbatov through for a second goal but his shot was deflected wide.
Southampton’s plight worsened in the 37th minute when Paterson was sent off.
The Southampton player, who had already been warned about a number of heavy challenges, was dismissed for violent conduct after a lunge on United defender Nemanja Vidic.
Moments later, Rudi Skacel was booked for a foul on Nani and Riley felt the full fury of the disenchanted home fans. The yellow card count increased in the 40th minute when O’Shea was booked after a foul on Davis.
Southampton’s cause was well and truly lost three minutes after the restart though when Riley awarded United a penalty for handball against David McGoldrick.
Oliver Lancashire brought down Welbeck on the edge of the penalty area and when Nani tried to get the ball over the defensive wall, Riley adjudged that McGoldrick had blocked its path with his arm.
The decision clearly a poor one as TV replays showed the ball hitting McGoldrick’s head as he turned away.
Nani took full advantage though when he converted the spot-kick – sending Davis the wrong way.
Davis then denied Welbeck a second goal when he tipped his shot over the crossbar in the 49th minute.
United’s dominance was such that they replaced Giggs and Carrick with Gibson and Rodrigo Possebon in the 55th minute.
In the 62nd minute, Welbeck made way for Wayne Rooney as United looked to set the seal a comfortable afternoon with a third goal.
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Tags: FA Cup, Manchester United, Rooney, Welbeck