Ronaldo was red-carded for head-butting Pompey midfielder Richard Hughes, but Ferguson claimed that the winger was provoked.
He told Sky Sports News: "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.
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.
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."