Goals from Danny Welbeck, Nani and substitute Darron Gibson earned the Barclays Premier League champions a simple passage into the next round.

But Sir Alex Ferguson's side were more than aided by the benevolence of Riley, who sent off Southampton's Matt Paterson for violent conduct in the first half and then harshly awarded United a second-half penalty.

Youngster Welbeck opened the scoring in the 19th minute when 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.

"It was offside," said Poortvliet after watching a replay in the tunnel. "The referee's assistant should see it because he is on the same line as the player."

The Southampton coach was also unhappy with Riley's decision to red card Paterson eight minutes before half-time - claiming it ruined the game.

The Saints youngster lunged at Nemanja Vidic and Riley showed him a straight red.

"I know the boy very well, it was a normal tackle," Poortvliet added. "I was disappointed in the game by the decisions.

"The boy made a tackle on the ball with one foot. He hit the ball but he sent him off, and that was wrong."

Poortvliet was also unhappy about the penalty decision just after half-time when Nani sent his free-kick straight against the arm of David McGoldrick.

"The penalty was a cheap one," he added. "The ball hit his arm but it is normal when you are in the wall.

"Even with 11 players it is difficult to play against Manchester United. With 10 it is more difficult but the boys worked hard and tried to do their best. They tried to get a good result.

"But it was not the case. United are a big team and they showed how to play football and what they can do with the ball.

"I only feel sorry that after the red card it was not a real game any more. It was a pity for the people and for my players because I think they deserved more than that."

Poortvliet believes his young side can learn from the footballing lesson they were given by United.

He added: "They will learn how to circulate the ball and how to lose their markers, they will learn a lot. For that reason I feel sorry because normally you should have a game until the end.

"But after the 1-0 and the sending off, it was so difficult to stay in the game. We did very well until half-time but then you get the penalty against you and the heads go a little bit down.

"My boys are unknown and maybe that has got something to do with it.

"Maybe he (Riley) does not know my players. You the decisions on both sides to be the same. That is what I missed today.

"But United deserved to win and I am not hiding behind the referee. It was just more of a game when it was 11 against 11."

On the sending off, United boss Sir Alex Ferguson told ITV: "It's certainly rash and maybe it's easy for the referee to send him off.

"I think maybe if the referee had booked in the first minutes then maybe it would have calmed him down a bit, because it was exuberance more than anything.

"He was charging about, and it happens sometimes, he was over-enthusiastic really.

"Maybe we could have done better but 3-0 is a good result against a good side."

Meanwhile United goalkeeper Edwin van der Sar is hoping this is the year he lands an FA Cup winner's medal.

The Dutchman told MUTV: "I haven't won it yet so hopefully it will be this year because my years are running out."

On the game he said: "We had a bit of luck with the first goal - but it should have been four or five I think.

"We have an interesting tie against Tottenham now, we are looking forward to that at the end of the month."