"I think this is a big season in Europe for Cristiano Ronaldo," Sharpe told the Manchester Evening News. "Once you get to the semi-final stage of the European Cup, a lot of it is down to individuals and match-winning players.
"Kaka was the key man for Milan last term; there is no doubt about that. He was different class. It was Kaka who beat United in the semi-finals. There wasn't really a lot of difference between the two teams apart from Kaka.
"I know it is a lot to ask, but if Ronaldo can up his game by two or three per cent to become a Kaka and dominate in Europe like he does in the Premier League, it will be great for United.
"I think that's what they need to make the next step, from being nearly there to making it to the final and winning the trophy again. Kaka is older than Ronaldo, so he is bound to be that bit more influential because of his experience.
"But Cristiano has four Champions League campaigns with United now under his belt. He always seems to be scoring and having a big influence for Portugal at international level.
"So he has very good CV himself now. He has been getting better and better, and as he gets older and wiser, then he will progress even further. I have no doubt that at some point he will become to United what Kaka is to Milan. He will get there in his own time. He could do it this season.
"He's grown up in the Premier League with United. We've seen the progress he's made in England to the point when he dominated everything last season and was the key man for the Reds.
"It could be the right time now to make that step up to have the same impact in the Champions League."