ENGLEWOOD, Colo. -- Denver Broncos rookie quarterback Drew Lock will open the regular season on injured reserve as the Broncos moved through cut-down weekend on the hunt for a quarterback to back up Joe Flacco.

Lock, who was a second-round pick (42nd overall) by the Broncos in this past April's draft, suffered a right thumb injury on his throwing hand in the Broncos' Aug. 19 preseason loss to the San Francisco 49ers. Before the injury, Lock, who is now wearing a cast on his right hand and wrist, had been moving toward being Flacco's backup as he tried to work through the usual rookie growing pains.

But it was not to be, as Broncos president of football operations/general manager John Elway said Saturday that Lock, tight end Jake Butt (knee) and running back Theo Riddick (shoulder) will open the season on injured reserve.

The Broncos expect it to take at least six to eight weeks for Lock to be ready to throw, and it could be longer given the injury is to his throwing hand. The Broncos could designate Lock for a return from injured reserve six weeks into the regular season and would then be allowed to have him practice for two weeks before deciding to either add him to the active roster or place him on injured reserve for the second half of the season.

Players on injured reserve cannot participate in practice, so from a developmental perspective, a full season on injured reserve for Lock would also mean a full season away from the practice field.

"We'll have to wait and see how it goes,'' Elway said. "The most important thing is getting the best 53. With where Drew is, obviously we would like to have him on the practice field, (but) he's got that thumb, it's going to be six or eight weeks, it's going to be longer with a quarterback with a thumb. We needed that (roster) spot.''

Lock's transition from Missouri's spread offense to the Broncos' offense has been a bumpy one at times, as he has battled footwork issues, as well as bouts of inaccuracy in practice. In the preseason, he was 31-of-51 passing (60.8%) for 254 yards to go with one touchdown and one interception.

Elway said he has met with Lock to outline what is expected of the rookie while he is on injured reserve.

"My discussion with him (Saturday) is it's not a year off for him, he's got to be able to do everything but be on the football field,'' Elway said. " ... Expect him to continue to work hard and get a good feel with everything, keep learning with offense ... be right in the middle of everything.''

Given the Broncos also waived both Kevin Hogan and rookie Brett Rypien on Saturday, Lock's move to injured reserve does put the Broncos in the market for a backup quarterback. Coach Vic Fangio joked Saturday that he was "loosening up right now.''

Fangio said: "We'll have it fixed -- we don't play for what, 12 days, 11 days; they'll be somebody in there.''

Elway said that because the new addition would have to be ready to back Flacco up in the Sept. 9 season opener in Oakland, it would help if the new quarterback had a least some experience in an offense similar to the Broncos' scheme. Broncos offensive coordinator Rich Scangarello runs a version of Mike Shanahan's and Gary Kubiak's West Coast scheme.

"Good thing is we have an extra day or two -- it is important to have that to have at least a feel for the offense,'' Elway said.