Ronaldo scored his 18th goal of the season on Monday night to keep alive the Red Devils' treble hopes and book an FA Cup semi-final meeting with Watford on April 14.

"We have had some great players at this club in my 20 years here and Cristiano is up there with the best of them," said Ferguson.

"He is phenomenal."

Real Madrid star David Beckham added to the plaudits saying he has been awe-struck by him this season.

The Manchester United winger has endured hostile reactions from rival fans furious with the part he played in Wayne Rooney's sending off for England against Portugal in the World Cup.

But Beckham, who was similarly unpopular for a time following his own dismissal for England during France '98, has been hugely impressed by his successor in United's No 7 shirt.

"What he had to go through at the end of last season was tough but he couldn't have been at a better team and under a better manager," said the former England captain.

"He's had the support of the fans and the club and, with that, you have every chance of coming through those bad times and he has been pretty incredible this year."

United win over Boro was tinged with controversy, with members of Gareth Southgate's backroom staff convinced Ronaldo had dived to win a penalty following Jonathan Woodgate's challenge in the 75th minute.

But Ferguson dismissed Boro's complaints and instead hailed Ronaldo's stunning contribution to the United cause.

I don't know what they (Boro) are going on about, it was a clear penalty kick.

"If it had been any other player they would not have bothered but they have a guilt complex about him.

"It is a shame. They are putting a stigma against the boy that he doesn't deserve."

Boro's frustrations with Ronaldo date back to the Premiership meeting between the two sides at the Riverside earlier this season, when the Portugal winger went down under little contact from Mark Schwarzer.

If that was not bad enough, United then received another borderline call in their favour in the initial quarter-final tie when George Boateng was ruled to have handled in his own box at point-blank range.

"The galling thing for me is that we have played them three times this season and conceded three dodgy penalties," said Southgate, who refused to condemn or apologise for his coaching staff's behaviour.

"In the heat of the moment, everybody is entitled to a reaction.

"It is the 85th minute of an FA Cup quarter-final. People are going to react in a way you wouldn't normally."

Southgate hardly helped ease tensions with his view on James Morrison's wild late lunge on Ronaldo which rightly earned the youngster a red card.

He added: "In that situation, we would all love to do that."

Southgate did concede his side lost their discipline at the end and, in a television interview which incensed Ferguson, Ronaldo was at a loss to explain why he is being targeted.

"I don't know why they carry on like that," he said. "Maybe it is because I am too good for them.

"It was the referee's decision to make but it was a penalty. That is why I went down."

Southgate also responded to the criticism of Sheffield United counterpart Neil Warnock, who slammed his decision to rest Jonathan Woodgate and Mark Viduka for Saturday's home defeat to Manchester City in readiness for last night's clash.

"Some people have had plenty to say about the team I picked on Saturday," added the former England defender.

"It was not just Neil Warnock, there were others as well. I make decisions for the benefit of this football club and I will pick whatever team I want."