The Canaries have already recruited six new players this summer, which in turn meant that the likes of Surman and Barnett had dropped down the pecking order.
"We're always very appreciative of the players here who have done well for the club in the past," the Eastern Daily Press quotes Hughton as saying.
"But of course what we also have to do is over a period of time make the changes that we feel are necessary and try to make the improvements in the squad that we can. That inevitably means players moving on, but it always has to be for the right reasons.
"I have to be honest with those players and sometimes it is difficult to say what you feel all of the time but with Andrew he had an injury that stopped part of his involvement and he is at an age where he wants to play more often. As is Leon."
Hughton's team open their Premier League campaign at home to Everton on Saturday, August 17.