During his news conference earlier in the day, coach Gary Kubiak was mum on whether Sanchez or Siemian would get the start, saying only that they would play during the first half.
As the only quarterback on the roster with starting experience, it's not a surprise Kubiak will start off with Sanchez. However, the 29-year-old has not pulled away from the pack in offseason workouts. Sanchez will have to fend off Siemian and first-round pick Paxton Lynch to be named the starter to open the regular season.
Siemian, a seventh-round pick in 2015, has impressed coaches throughout the offseason. Lynch owns the most raw talent of the trio, but needs to show progress with the playbook and managing the huddle. Still, it wouldn't be a shock if the rookie were able to leapfrog up the depth chart with strong preseason performances.
Kubiak noted Monday he's not close to determining a Week 1 starter.
Sanchez will get the first shot Thursday to prove he can win the regular-season job, but this competition is far from over.