Schalke 04 boss Roberto Di Matteo has insisted that his side must be wary of Cristiano Ronaldo in Wednesday's Champions League last-16 first leg.

Ronaldo has netted 36 times in 33 games this season, but has now gone three games without a goal in the league, with his form being called into question since the turn of the year.

However, Di Matteo has claimed that the Portuguese is still capable of 'winning a game by himself'.

"Cristiano is a phenomenal player who is always capable of deciding a match on his own," Di Matteo told reporters. "Even if his form is not as stellar as usual, he can still win the game by himself."

It has also recently been suggested that a long-standing knee injury is impacting Ronaldo's form.