"I played with the national team all the way to under-21 level. Who did I play with? Wesley Sneijder, van Persie, Rafael van der Vaart, Johnny Heitinga . . . most of the ones who are now in the senior squad. I knew I was not ready to make the step up, but I feel I am getting better and coming closer to the level required. Let's hope I can take that step at Celtic."
Hoops boss Gordon Strachan said: "He'll be a good signing for us.
"We want to win everything and to do that we need a good squad. He is a good age, years away from his peak, so we have to prepare him for that and to do well in the Champions League.
"He will gain experience and get better. We do not have £20m to spend: if we did, he wouldn't be sitting here, it would be Rio Ferdinand."