I believe Tilley is contracted until either 2017 or 2018, I'm pretty sure I read something after or before the last game at Boro last season stating that he's been tied down to a longish deal.
From the information on AZ's contract page on here, these are the DS players whose contracts are due to expire at the end of the season:
Adam Chicksen, Daniel Akindayini, Ben Barclay, Dylan Barnett, Will Collar, Tom Dallison, Jason Davis, Robin Deen, Harry Doherty, Rob Hunt, Dessie Hutchinson...