Devin Singletary is a darn good RB. He is averaging 4.7 yards for his career and you saw what he can do when the Bills OL was stabilized later in the year.
So, the Bills only need a backup RB that can catch the ball and pass block. Duke does both well.
But I will bet that the Bills draft a RB somewhere late in the draft.
Anyways, this is Allen's team and the Bills won't be running much.
As long as the OL is stable, the Bills can easily get by with what they have at RB right now.