I think there is no way Modric is becoming a regular starter at Milan. He can't handle such a tempo anymore truthfully. This guy is soon 40 years old now...
Maybe one can say Ronaldo is still like that at the same age. But he is Ronaldo. He still has a stunning physique which helps him keep a good form in matches even now.

So coming back to the main topic, Modric would be more like a mentor there, yeah.
Modric few times played for complete 90 minutes when he was play for Real Madrid.
He might be not able to play for complete 90 minutes all the time, but I'm sure he can, the tempo in Serie A is slower. You might think like that because Modric's physique doesn't look as good as Ronaldo, but he has still a good stamina.
Let's see whether he can change Milan or not, Milan is a like curse team now.