Carlo Ancelotti's men have shipped seven goals in their last three matches in all competitions, while they have failed to keep a clean sheet in their last four attempts.
Even though Real have conceded just two goals in three Group B fixtures, France international Varane believes there is scope for the Spanish giants to be more resilient at the back.
"I think we can improve in defence but that's something we need to do together," he said.
"We need to improve in defence but that's not only a defender's concern but all the team. I think if we all take this into consideration we will improve."
Antonio Conte's Juventus could find themselves five points adrift of second place with defeat at the Juventus Stadium if Galatasaray beat Copenhagen in Denmark.
And Varane believes Real need to be on their guard against a side hoping to maintain their hopes of qualifying for the knockout stages.
"They need the victory and they will work very hard. They will be tough and try very hard to win and we need to be ready for it," he continued.
"We need a point to qualify, but we want first place in the group. They need a win and there is going to be a high pace to the game and we must respond to that."
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