Funny thing is mate, he is one of the players that you could wave the medical with practically, never missed a single game with an injury throughtout his career, something that has been overlooked in how good a deal it is to get him, Witsel similarly has a stellar fitness record as well, with i believe only one injury since he started playing and that only for a few games.
Not something i have checked Koemans history out on with transfers but interesting if he does go and target very durable players in the market to build the spine of the team around.