The England Under-21 player said: "Johan is a good player and I've got so much respect for the guy. I've known him since about 15 when I was a schoolboy at Arsenal.
"He just seems to get better and better. He has been applying himself well for Birmingham and I am sure the gaffer will be trying his best to keep him here - and we hope he stays.
"We are good friends but he has got to do what is best for him. The last time we had a conversation about it his mind was not sure what to do.
"But I'm sure when you have had a taste of regular football in the Premier League, you don't want to go back to playing one week and not playing the next."