1-2 years learning under Koeman first.. before what? He' eligible for a move to United/Chelsea?
Top clubs don't just buy ready made talent, I'm sure you're aware. They buy potential, and he has bags of it. If anything, after a less than spectacular season, now would be the perfect time to go in for him again.
Give him 2 years with Koeman and he could be a world class player and the fee will then be astronomical.