Ronaldo claimed his second consecutive Fifa Ballon d'Or after a fantastic 2014 in which he fired Los Blancos to Champions League glory, as well as breaking a succession of goalscoring records, both in Europe and Spain, over the course of the year.

However, the Portugal international's current form and conduct are currently topics of debate, with the forward having gone three games without finding the net in La Liga, and his decision to go ahead with his 30th birthday party celebrations just hours after a humiliating 4-0 loss at Atletico, which upset some fans.

Di Matteo acknowledges that there has been a minor dip in Ronaldo's performance level, but the Schalke boss insists that he will not be underestimating the threat posed by the Blancos ace at the Veltins-Arena.

"Cristiano is a phenomenal player who is always capable of deciding a match on his own," the Italian coach told reporters on Tuesday.

"Even if his form is not as stellar as usual, he can still win the game by himself."

Despite Ronaldo's relatively sub-standard performances of late, Madrid are widely expected to sweep Schalke aside, just as they did at the same stage last season, when they routed the Bundesliga outfit 9-2 on aggregate.

However, Di Matteo insists that his players believe that they can upset the reigning champions of Europe.

"Our goal going into the match is to come away with a win," the former Italy international added. "We want to exploit the few weaknesses in Real Madrid's game.

"Of course they are the favourites. They are the holders, but still, we are out to win. 

"It will be a test to see how far we've come in the last weeks and months."

Di Matteo succeeded Jens Keller as Schalke boss last October and the Gelsenkirchen outfit currently sit fourth in the Bundesliga.