Southampton captain Kelvin Davis believes Matt Paterson will learn from the red card shame that cost them any chance of an FA Cup upset against Manchester United.

The Saints were already a goal down to Danny Welbeck's 19th-minute opener before Paterson was red-carded by referee Mike Riley for a lunge on United defender Nemanja Vidic.

Southampton head coach Jan Poortvliet was upset by Riley's decision to dismiss the 19-year-old eight minutes before the interval but United manager Sir Alex Ferguson claimed the youngster paid the price for his "over-enthusiasm".

But Davis, who kept the scoreline down with a string of superb saves, said: "When you go down to 10 men against Manchester United then you know it is going to be very tough.

"But he is a young lad and hopefully he will learn from it. He went for a ball that maybe wasn't there to be won but he is an honest lad and certainly would not have meant to hurt anyone."

United boss Ferguson said Riley could have helped the situation by speaking to Paterson in the opening minutes when the Southampton player made a number of heavy challenges that went unpunished.

"It was certainly rash and maybe it was easy for the referee to send him off," said Ferguson.

"I think maybe if the referee had booked him 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."

But unhappy Poortvliet was upset with Riley's decisions all afternoon.

He claimed United's first goal was offside, that Paterson should not have walked and United's second-half penalty, awarded for handball, was a "cheap" decision.

"The first goal was offside," said Poortvliet. "The referee's assistant should see it because he is on the same line as the player."

But Poortvliet did not stop there. He was also unhappy with Riley's decision to dismiss Paterson.

"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."

When Nani hit his free-kick straight against David McGoldrick's raised arm, Riley was the subject of further criticism.

"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."

Youngster Welbeck opened the scoring in the 19th minute when Nani's corner was headed against the crossbar by John O'Shea and the striker had the simple task of nodding home the rebound from point-blank range.

Nani scored United's second from the penalty spot and substitute Darron Gibson smashed home a late third to notch his first goal for the club.

Boss Ferguson was full of praise for impressive teenager Welbeck, who is now showing distinct promise at Old Trafford.

"Danny scored and did fine," said Ferguson. "He has got a good temperament and personality and he enjoys playing.

"He is still growing, so there are periods when he gets tired but the potential is definitely there."

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