Ronaldo was red-carded for head-butting Pompey midfielder Richard Hughes as United drew 1-1 in the Premier League at Fratton Park, but Ferguson claimed that the winger was provoked.
He said: "He was surprised at the decision. But Steve Bennett has sent him off before, and I think he took great delight in sending him off too."
However, Ferguson reserved some criticism for his own player.
"He was provoked and he fell for it. He fell into a trap and has to learn from it but he received no protection all night."
Ronaldo appeared to aim the butt at Pompey substitute Hughes during a goalmouth melee in the 84th minute - just seconds after Portsmouth new boy Sulley Muntari was dismissed for a second bookable offence.
While Muntari will miss just Saturday's home game with Bolton, Ronaldo faces a three-match ban that puts him out of clashes with Manchester City, Sunderland and Tottenham.
Ferguson, who is also without the injured Wayne Rooney for two months with a broken foot, told MUTV: "This is something Cristiano has suffered from a few times. I remember Bennett sent him off against City for striking back."
Ronaldo was previously sent off against City and Aston Villa after joining United from Sporting Lisbon in July 2003 and Ferguson admitted: "He has got to learn from this.
"He has got to tell himself that inferior players are trying to stop him and not fall for these things. That is why they are doing it.
"But he has fallen into the trap of intimidation and that's something that has happened a few times.
"Portsmouth got stuck in at times in the second half and we suffered some rough treatment. You just have to show a bit of calmness when that happens.
"But Ronaldo and Carlos Tevez got no protection all night."
Tevez made his United debut as a lone striker and set up the opening goal for Paul Scholes after 15 minutes. The former England midfielder rifled in his 96th goal for the club.
Ferguson said: "He was excellent and that was one of the good things to come out of the night for us. We are already without Rooney and to lose Ronaldo for three games in a big blow."
Portsmouth levelled in the 53rd minute when Benjani Mwaruwari timed his run from deep to perfection to power a header home from Matthew Taylor's left-wing cross.
United, who drew 0-0 at home to Reading on Saturday, face Manchester City on Sunday chasing their first win in six competitive games - not counting the Community Shield which they won on penalties.
Pompey boss Harry Redknapp admitted he did not know why Ronaldo was sent off.
He said: "I saw a red card flourished and just hoped it wasn't another one for us.
"We are now down to 16 players for next Saturday with Sol Campbell and Noe Pamarot (hamstring) struggling and Linvoy Primus miles away at the moment in his recovery."
"We will miss Muntari as well. It was two yellow cards, the first an innocuous trip when he chased Ronaldo and later when he went in a bit late. He'll have to watch it in English football."
Redknapp had to do without new skipper Campbell after he damaged a groin muscle in Saturday's 2-2 draw at Derby.
Right-back Lauren injured an ankle in training yesterday and Dejan Stefanovic is also sidelined.
Redknapp said: "It was a fantastic performance. I would have certainly settled for a point tonight before the game.
"Anything you take off United or any of the big four teams has got to be a good result.
"It has always been our trademark. We have a go and that was a typical Portsmouth performance.
"Although United scored in the first half, they didn't make that many chances and David James didn't have to fly about making dozens of saves.
"But I changed it a bit in the second half, pushed Benjani right up front and played two other attacks wide and it worked out.
"United are a great team. They always seem to have five or six different options on the ball and Giggs pops up everywhere.
"But we regularly take points off them here and we were well worth one tonight.
"Benjani scored a great equaliser and the crowd love him for his work-rate and ability. His only failing has been scoring so few goals but he's got two in two games now and hopefully can put that right."
Ferguson added: "We played some fantastic football and had to deal with some of their rough stuff but we were also guilty of failing to put away our chances."