The 20-year-old signed for the Blues on Wednesday in a deal believed to be in the region of £25 million and has already impressed on British shores with his performances at the Olympics.
Playing a starring role as Brazil beat Egypt in their Group C opener, Oscar created two goals in the 3-2 win in Cardiff and Rafael, who opened the scoring in that game, rued the fact that United's Premier League rivals had secured his signature.
Reflecting on Oscar's assist for his goal, Rafael told Fifa.com: "Wow, did you see that? Oscar’s just too much!
"He’s an incredible player. When I see him doing these amazing things it almost makes me feel sad to think that he’s gone to one of our rivals."
The Selecao face Belarus at Old Trafford in their second game on Sunday and Rafael is looking forward to a positive reaction from the home crowd.
"Well, I’ve been talking about that with the rest of the guys in the dressing room, telling them: 'I’m going to play at home and you’re not,'" he said.
"I’ve been there for five years now. I know everyone and I think a lot of the fans who’ll be going to the stadium will know me well. I think the reception I’ll get will be different."
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The former Fluminense youngster will face stiff competition from the likes of Daniel Alves and Maicon for a place in the Brazil squad for the 2014 World Cup although the 22-year-old is confident he can make the cut.
He added: "Playing for Manchester United gives me hope that I’ll be there at the World Cup. I’ve always said that.
"I know I’ll have a lot of competition from some great players like Dani Alves, and I know how important it is to do a job for Mano [Menezes] whenever I’m playing for A Selecao. But I think I can do it."
Twin brother Fabio recently joined QPR on loan and, despite having spent his entire career thus far playing alongside his sibling, Rafael believes the time apart could be a good thing for the duo.
"In one way, it will be good for us to be apart," he continued. "Perhaps it will help us mature a little more.
"It could end up being a good thing for us. It could give us a different outlook on things. After all, we’ve been together for 22 years now. Fabio is happy there and I’m happy here, with the national team at Old Trafford. I’m going to be thinking of him, that’s for sure."