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.
