Ronaldo only scored his first league goal of the season during Saturday's 2-1 victory over Getafe, having missed the opening four weeks of the campaign through suspension and then failing to find the back of the net in his next three outings.
The four-time Ballon d'Or winner has been more prolific in the Champions League so far, though, scoring four goals in two games, and Isco is confident that the Portugal international will be at his best for the visit of Spurs.
"It was only a matter of time. He is a killer. It's true he hadn't scored many goals in the league so far this season but in the Champions League he has. It is very good for us that he is off the mark in the league because we need him," he told reporters.
"This team has no limits. We want more titles. As soon as we have our whole team available, we will see a stronger Real Madrid."
Ronaldo has scored eight goals in 16 previous meetings against Tottenham.