The young goalkeeper has been impressive in Germany for the Oranje.
Stern told the club's website: “He’s a 21 year old, but he doesn't play like a 21 year old. You could be speaking to a guy who's 27, 28. He's got a very strong head on his shoulders and that has helped him so much adapting at such a young age to the Premier League and now, of course, at the European Championships. Without that maturity I don't think he'd be doing as well as he is right now.
“One concern that we had last season was do we overexpose Bart if we throw him straight in when you either sink or swim. We're now seeing the reward for that. I'm sure he wanted to play a lot more games, as did Jason Steele, but it helped his development and I think he's in a good place now because of that.
“His focus and the way that he prepares for games is top level.
“There's never any more pressure than when you're playing for your country at a major international tournament, especially towards the final stages, so for him to learn at a very young age how to deal with that and to manage it well is brilliant.
"He's got great people around him to help support him in those situations, and I think his experiences in Germany can accelerate his development even quicker.”