Ronaldo saw red after a clash with Portsmouth midfielder Richard Hughes.
Fergie said: “Cristiano is responsible for falling into the trap of intimidation.
“It has happened to him a few times and it happened again tonight.
“I looked at the replay and it was a bit inconclusive but my take is that he was provoked and he has fallen for it.
“Cristiano has to remind himself that he is a better player than these players and you have to react with a bit of calmness.”
