The Swede was outstanding on-loan at Feyenoord this season.
"He's the same level that Wesley Sneijder and Van der Vaart were at the same age," enthused Koeman.
"John was impressive as soon as he started playing with us. To do that at 19 shows you're important. For that reason, I'd say he is as good as (Zlatan) Ibrahimovic."