Real Madrid forward Cristiano Ronaldo has vowed not to celebrate if he scores in the Champions League round-of-16 tie against Manchester United.The Portugal international enjoyed a six-year spell with the Premier League leaders before leaving them for los Blancos in the summer of 2009, and he still has warm feelings for his former club."I will not celebrate my goal if I manage to score at Manchester United," Ronaldo was quoted as saying by AS after Thursday's draw.The 27-year-old recently already expressed his gratitude towards Sir Alex Ferguson, and stressed that the United manager has played a key role in his development."[Ferguson is] a fantastic person. A human person. He taught me many things," Ronaldo told The Sun."Like I said before, he's like a father in football. I really miss him and my relationship with him."Madrid start the tie with a home game on February 13, before Ronaldo returns to Old Trafford for the second leg on March 5.