Ronaldo scored the series-winning goal with United reduced to 10 men after his international teammate, Nani, was sent off for a controversial red card. Madrid advanced with the 2-1 victory (3-2 on aggregate) and awaits the quarterfinal draw to determine its next opponent.
"I did my job - helping Real Madrid to win - and I'm very happy, but it's a strange feeling. It was the sensation of wanting to do something but not being able to," Ronaldo told TVE.
"I played at Manchester United for many years, and if someday I face Real Madrid it will be the same. To be received well is special for me, coming home, seeing friends. Inside I'm happy but also sad for Man Utd.
Like after his goal in the first leg of the tie, Ronaldo did not celebrate his goal out of respect for his former club, despite the massive importance of the tally.
"I didn't celebrate out of respect to those who treated me well here, but what I wanted the most was winning and keeping Real Madrid on course for La Decima," Ronaldo said.