If the writers book Sandow to successfully cash in on Cena, then he will. If the writers book Sandow to not successfully can in on Cena, then he won't. Though, will he be booked to cash in successfully on Cena? I could see it happening honestly.
Anything is believable with the proper set-up. Too bad he'll probably be the biggest joke of a World Champion if/when he successfully cashes in. It will further devalue the belt down to comedic levels.