It was Ronaldo's 36th goal of the season and United went on to defeat the Italian title-chasers 2-0 - with Wayne Rooney extending their lead.
Ronaldo has taken his great domestic form into the European arena and now scored seven goals in eight games.
However, he can recall little of his latest effort as he was left dazed and needing brief treatment.
Ronaldo said: "To be honest, I don't remember much about the goal.
"I went down and I felt pain. I didn't enjoy it.
"But I will have time to watch it on television - others say it's a good goal."