Nothing, but that's new thinking which people can't accept in the NFL. Manuel is a ~$2M cap hit for the next 3 seasons. That's cheap even by backup standards. There is no reason to avoid having a QB competition, even if Manuel was good.
If you can draft a QB that you feel great about, you do it. Period. They're that valuable today.
Of course, there's a 1% chance this Bills staff does so, so this is pedantic. But, one can hope. I always do. I'm a Billiever.