Hammer hasn't beaten anyone decent in seven years and will be down six rounds on the cards when the fight starts. He is also shot. Zero chance he wins
Hammer gave up on being competitive years ago. Chisora still got the mongrel in him and always will. Probably to his detriment now.
As others have alluded to, Hammer is just someone you bring in to give you rounds at this point, and there is no guarantee of that any more.
Hammer sells himself at this point. You have to go back to the Price fight for the last time he was a live dog. Chisora easily wins this.