The reigning Champions League holders go into the second leg of their Champions League last-16 clash with Ajax smarting from successive home losses to Barcelona, who sit 12 points above them at the top of La Liga.
Santiago Solari's men have lost three of their last four matches in all competitions, and Modric was asked whether the absence of Ronaldo, who joined Juventus before the season began, was taking its toll on his team-mates.
Modric told reporters: "He's a player every team would miss. Looking for a replacement for Cristiano is almost impossible. What he's done for this club... of course we miss Ronaldo.
"What the club tried was for other players to fulfil his role, which was divided between the attacking players. And that's not easy. Cristiano scored 50 goals and you can't find someone who scores so many.
"Some had to take a step forward and, not scoring 50, but for three players to score at least 15-20 or 10 goals. And we don't have that. That's why I think it's our biggest problem this year. For example, in the Copa against Barcelona, we had a lot of chances but an opponent with so much quality punishes you."
Modric pointed to the wealth of attacking talent already in Madrid's squad and said the club must unite around them to ensure a difficult campaign ends on a high.
The 2018 Ballon d'Or winner also backed Bale - whose agent launched a stinging attack on fans who booed the Wales international during Saturday's Clasico defeat - to return to his best after a torrid period in which his future has been the subject of media speculation.
"Cristiano is not here but we can't complain about that for 10 years," said Modric.
"The club has put faith in other players like Gareth [Bale], [Marco] Asensio, Karim [Benzema], they signed Mariano, Vinicius [Junior] is here and he's doing really well for his age.
"[Bale] is okay. Sometimes a player enters a phase of their career where they don't feel better. Everyone goes through it. I did in the past.
"He looks happy and excited. We can't forget what he's done for this club, things that are very easy to forget.
"He wants to work and he wants to get back to being the usual Gareth. He has a lot to give to this team. He has to work, work and work."