Just as you have made mentioned i was also thinking of this that Luka Modric injury risk at Ac Milan, given his age 39, pulse i still think AC Milan to manage his playing time because despite his age modric experience and skills will be valueless assets for AC Milan, how ever i think the team should prioritize his fitness and manage his playing time to minimize injury risk. And aslo think he had still shown he can still preform well even at his age, i think in recent debut with AC Milan, he came off the bench in the 65th minute during the copper Italian league match against Bari.
Even while at Madrid, Modric's play time was given considerations and well managed such that his injury risk exposure was reduced, you are very correct, if AC Milan wants to get the best of Modric, they will have to pay good attention to this if not it they will have him injured and they will not get the value for the money they have spent on this transfer. By the way.they just signed Boniface to the team this will further help strength the team and possibly reduce the stress chance on othe players like Modric