the only thing that can give us a definitive dx is a mri in this case if it is indeed a UCL sprain. If he didnt have an MRI by now, id be very surprised so i doubt thats the case.
we can only really speculate on is why the Bills arent telling us the dx, is it for competitive advantage against the vikings? is it because it was negative and they are hoping he can deal with some pain through out the week to play? or is it because they are afraid to break the news that he indeed needs to sit out some games. I dont think its the last one, bc news like that breaks out rather fast.