Head coach Zidane hopes the injury is only minor but is uncertain how long Modric will be out of action.
"Modric has a hamstring issue," he told a news conference. "I always have faith in the staff here, the physios and doctors, so he will be back with us quickly. I don't think it is serious."
The injury is the latest to hit the Madrid squad since their 3-1 win over PSG in the first leg of the Champions League last 16 last week.
Toni Kroos has picked up a knee problem and Marcelo limped out of Sunday's 5-3 win over Real Betis with another suspected hamstring issue.
Zidane remains hopeful, though, that all three could return for the meeting at Parc des Princes.
"I am always positive," he said. "I do not like to see today that we have three or four players out injured. I hope they are not out too long, but we will see. I hope it is short.
"It can be that, when there are certain [injury] problems, it's because of an enormous effort. But, at the same time, in order to cope with matches like the one against PSG, we have to repeat those efforts.
"We have to maintain the pressure in every game and to keep doing it for 90 minutes. And doing it every three days is positive.
"When you're winning playing every three days, you're more optimistic. That's what we need now."