O'Brien said Belton remained "day-to-day," but Day will now get the nod even if the sophomore's ankle improves. Belton was sidelined in the third quarter against Ohio, and an ice pack was wrapped around that left ankle.
Belton did not practice Wednesday when the media arrived during an open 30-minute session, and Day took reps with the first team.
Day, a fifth-year senior, has just 15 career carries and was listed as the No. 3 tailback in the spring. With Silas Redd's departure and Belton's injury, he now climbs to No. 1.
"He always gives his best effort and I'm real excited to watch him play and for him to get that opportunity," linebacker Michael Mauti said earlier this week. "I know he's going to make the most of it."