Both players returned to first-team training on Sunday after missing the 3-1 La Liga win over 10-man Getafe at the Santiago Bernabeu that saw Cristiano Ronaldo net a brace to help Zinedine Zidane's side bounce back from a shock defeat to Espanyol and close the gap on second-place Atletico Madrid to seven points before their cross-city rivals lost to leaders Barcelona.
"They still haven't trained with us, we'll have to see if they do tomorrow," Zidane said after that victory, per the Mail. "I will never rue having players absent.
"They are important players but we have to look to Tuesday's game. If they can't be with us, then someone else will play.
"We have two days to see how Luka and Toni will be."
Neither former Tottenham Hotspur schemer Modric or German international Kroos have featured for Real since that 3-1 first-leg defeat of PSG in Madrid on 14 February, with the former left as an unused substitute in an eight-goal thriller against Real Betis before missing meetings with Leganes, Alaves, Espanyol and Getafe.
The latter, meanwhile, was said to have sprained the exterior lateral ligament in his left knee during a first-half collision with Kylian Mbappe and played on for an hour, completing the full 90 minutes.
Both Kroos and Modric did accompany their teammates on a flight to Paris on Monday, with Zidane able to name a full-strength travelling squad after defenders Marcelo and Jesus Vallejo also resumed training last week following respective hamstring injuries.
However, whether either is deemed fit enough to start alongside Casemiro remains to be seen, with Mateo Kovacic limited to second-half substitute's cameo against Getafe at the weekend as a likely contingency measure.
12-time winners Real's attempts to preserve their advantage in Paris and seal a spot in the quarter-finals as they seek a third consecutive Champions League triumph has already been boosted by the absence of Neymar, who underwent an operation on the fractured fifth metatarsal in his right foot in Belo Horizonte, Brazil on Saturday and is expected to be sidelined for around three months.
Fellow PSG forward Mbappe is also doubtful for the second-leg clash at the Parc des Princes with an ankle problem suffered during the Coupe de France quarter-final victory over Marseille that kept him out of the 2-0 Ligue 1 win at Troyes.
Goalkeeper Alphonse Areola returned to a weakened squad for that fixture, though centre-back Marquinhos remained out with a thigh issue.