Going through their squad, the only players I recognise from last season are:
Worrall
McKenna
Cook
Colback
Yates
Arter
Cafu (no, not that one)
Johnson
Surridge
Walker
Looking through that list, I think Worrall, McKenna, Yates, Colback and Johnson were regular starters for them, so they have lost a massive chunk of their squad for one reason or another.
Neco Williams and Johnson won't have to be registered as they are 21, not sure about Gibbs-White, O'Brien and Biancone
Edit: Just looking at Transfermarkt, and their total minutes by player is interesting:
Johnson (Stay)
McKenna (Stay)
Yates (Stay)
Worrall (Stay)
Samba (Gone)
Spence (Gone)
Garner (Gone)
Colback (Stay)
Zinckernagel (Gone)
Grabban (Gone)
Figeueiredo (Gone)
Lower (Gone)
Cook (Stay)
Davis (Gone)
Lolley (Gone)
Those are all players who played over 1000 minutes. 6 are still with them, 9 have left for one reason or another.