The two countries will meet in Durban on June 25 in the last Group G match of the tournament which runs from June 11 to July 11.
"It will be a special game because I am Brazilian, a totally different game from all the others. I chose to play for Portugal and I will defend that shirt until the end. When I made the decision I knew these matches could happen," the midfielder told the A Bola sports daily.
"If I score, no Brazilian can see that as a betrayal against the country in which I was born."
"Given my career, the most normal thing would have been to play for Brazil. There are players who didn't achieve half of what I have, but who wore that (Brazil) shirt," he said.
"Playing in the Brazil team would have been the easiest and most natural thing for someone who won everything with Porto, was in Barcelona's starting XI for four years and plays at one of the main clubs in the world (Chelsea)."
"I believe, honestly, that I am going to have a great World Cup", he said.
