I don't think it makes very sense for Milan to sign Modric. He is soon becoming 40 years old. How much can he contribute to the team? I respect his career very much but I think it is time for him to think of retirement.
I mean he can't play full time. Probably Milan wanted to benefit from his experience by subbing him in later in a match. I don't expect him to be a regular starter.
Because his match condition wouldn't be convincing to play effective for 90 minutes.

I think that when it comes to physical fitness, Modrić can still play the full 90 minutes. Of course, his form isn’t what it used to be, but unfortunately, the italian league isn’t as demanding as it once was, and players generally have enough time to adapt. His experience will bring many benefits to Milan, especially since they’ll most likely be fighting for the Scudetto. Never underestimate Luka we’re talking about the former number 10 of Real Madrid
