Head coach Gary Kubiak told reporters: "He continues to grow, and we're struggling and looking for a spark. I think he deserves an opportunity to go out there.
"I wanted to operate today and tomorrow with him getting the majority of the reps and the team knowing what's going on. He's given me confidence throughout camp.
"I'm not sending him out there by himself. I'm sending him out there with this football team, and the guys understand that."
Keenum, who will be making his first start in the NFL, joined the team as an undrafted free agent last season.