Southampton 0 Manchester United 3: Manchester United set up an FA Cup fourth round home tie against Tottenham with a comfortable win over Championship strugglers Southampton.But Alex Ferguson's side were more than aided by the benevolence of Mike Riley.

The referee sent off Southampton's Matt Paterson for violent conduct in the first half and then incorrectly awarded United a second-half penalty.

Goals from Danny Welbeck, Nani and substitute Darron Gibson 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.

The gulf in class was beginning to show as the game settled with Anderson and Berbatov providing some moments of real quality.

And the goal United had been threatening finally arrived in the 19th 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.

Southampton's plight worsened in the 35th 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.

Welbeck almost snatched a second at the near post in first-half stoppage time but Davis once again reacted well to tip the ball around the post.

Southampton's cause was well and truly lost a minute 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.

United's dominance was such that they replaced Giggs and Carrick with Gibson and Rodrigo Possebon in the 55th minute and the visitors were rewarded with a third goal in the 81st minute when Gibson smashed home a cross from Rooney.