Madrid take a 2-1 lead into Tuesday night's second leg of their last 16 clash against Ajax but their attacking potency has been called into question following a 1-0 defeat on Saturday in La Liga to Barcelona.
The loss left them 12 points behind the pacesetters with 12 games remaining.
And after the same opponents beat them 3-0 four days earlier in the semi-final of the Copa del Rey, the Champions League remains Madrid's last chance of silverware this season having already claimed the Club World Cup in December.
Modric, who succeeded Ronaldo as winner of the Ballon d'Or, admitted the move to replace the Portuguese star with several goal scorers had not gone according to plan.
"Not having Cristiano Ronaldo would affect any team in the world," said Modric. "It's almost impossible to replace him so of course we miss him.
"The club has confidence in players like Gareth Bale, Marco Asensio and Karim Benzema. Vinicius Junior is doing well given that he's only 18.
"But things don't always go according to plan so the thing to do is to keep working and stay together. Real will get back to the top as it always has done."
Madrid go into the game at the Santiago Bernabeu without skipper Sergio Ramos. The 32-year-old will serve the first of a two game ban after receiving a third yellow card during this season's tournament.
Normally that is met with a one match suspension but Uefa, which runs the Champions League, increased the punishment after Ramos had suggested he deliberately got himself booked in Amsterdam so that he would be clear for the first leg of the quarter-final.
"We're pleased that we've got the Champions League," said Modric. "Losing to Barcelona twice in such a short space of time to Barcelona doesn't make you feel that great but we don't have time to feel sorry for ourselves.
"We've got to concentrate on the Champions League and try to win against a team that has nothing to lose."
While Madrid are seemingly perennial fixtures in the latter stages of the most prestigious competition in European club football, Ajax have not reached the last eight since 2003.
"Madrid keep going on about not scoring goals but they scored two against us in the first leg," said Ajax defender Nicolas Tagliafico. "We had lots of chances in Amsterdam but we could not put them away.
"They didn't have as many as us but they beat us. We can't get into the hype that they can't score goals. We have to think about taking our chances."