"I'm good friends with Cristiano and he told me about the club and what a great place it is," Anderson told ManUtd.com. "Joining United is a dream come true for any footballer in the world. I'm really excited to be here and hope I can do well.
"I was doing well at Porto at the time and I knew that joining a club like United - one of the biggest clubs in the world with one of the best followings in the world - would bring its own pressure.
"But when they expressed an interest in me again in the summer I felt it was the right time and I was really excited to have the opportunity to play for such a great club.
"I can't wait to play my first game at Old Trafford," he added. "It will be amazing, not just for me but for my family as well.
"As a young boy it was always my dream to play for a big European club. I was lucky enough to do it with FC Porto and now I'm at United. I feel proud to be here and I want to do my best."