The back-to-back European Cup winners are now left battling for silverware on just one front after exiting the Copa del Rey at the hands of minnows Leganes in midweek.
On the back of a memorable year when winning a European and domestic double - the first time they have done so since 1958 - Madrid's 2017-18 is on the brink of disaster as they are also 19 points adrift of La Liga leaders Barcelona.
Ronaldo, who himself has yet to reach the heights of last term, believes that his side can still salvage their season should they once again go all the way on the continent.
"If we can win the Champions League at the end of the year, it will be an incredible year," he told Chinese football app Dongqiudi. "We haven't started well in La Liga and we are disappointed, nobody is happy about that.
"But now we must work hard for the Champions League because it's the most important trophy for us. It will be difficult in La Liga, but we can't give up, even if we have dropped many points since the start of the season. The only solution is to keep working."
Madrid face a tough test if they are to advance beyond the last-16 stage in Europe, having been drawn against Paris Saint-Germain.