With the Denver Broncos' top two quarterbacks sidelined by injuries, rookie Tom Brandstater will start the team's preseason finale Thursday night against Arizona.

The start, even though it comes in the last exhibition, represents a dramatic turn for Brandstater, who watched from the sideline barely taking a snap in practice let alone the exhibition season as Kyle Orton and backup Chris Simms got virtually all the work.

Brandstater said he's spending extra time studying this week to get a better handle on the playbook going into the game.

"As a starting quarterback now _ that actually sounds crazy to say _ but I am starting on Thursday, and there's a lot of work that needs to be put in to be the guy," a bemused Brandstater said. "While the other guys are not here, I'm going to act like I'm the starter, and I'm not going to look back until someone tells me I'm not playing anymore."

Brandstater, a sixth-round pick out of Fresno State, played more than a half in Sunday's 27-17 loss to Chicago, leading the Broncos to two touchdowns after coming on for the injured Orton.

Orton gashed and apparently dislocated his right index finger when he struck a defender's helmet with his passing hand during a follow-through.

The Broncos were already without Simms, who suffered a left high ankle sprain during an exhibition game the week before at Seattle.

Coach Josh McDaniels said he remained hopeful that both Orton and Simms would be able to return to practice next week and be available for the Broncos' Sept. 13 opener at Cincinnati. But he also left open the possibility that one or both could miss the first regular season game.

"We're hopeful we'll have all our players ready to go on opening day, but again the timetable for that, I would be totally guessing and probably wrong if I started guessing about that stuff right now," McDaniels said.

Neither Brandstater nor fourth-stringer Ingle Martin has had a meaningful snap during the regular season.

McDaniels said Brandstater had a strong practice Tuesday _ the team's only major workout prior to the preseason finale _ and his grasp of the offense has improved significantly.

"He's a mature kid and he's really picked up on some of the things that sometimes take quarterbacks longer to do, run the huddle, get the formations lined up and that kind of stuff," McDaniels said. "He's getting better every day and that's what we like to see out of our young guys."

Brandstater saw his first action as a pro when he replaced Simms on short notice and looked overwhelmed, taking four sacks and completing one of his four throws for 1 yard against the Seahawks. But he appeared much more poised in his most recent outing, completing 8 of 12 passes for 110 yards, including a long of 54 yards to fellow rookie Kenny McKinley.

Brandstater said he can't really explain the leap in his play.

"I've tried to analyze it myself and I have no idea. Really, I'm still in the clouds, trying to get better," he said.

Brandstater did say that his confidence has grown since experiencing some success in his last outing.

"Something clicked," he said. "I felt so much better, and I don't even know when it was, but at some point, I felt like I really belong out here. I can do it. There wasn't that nervous second-guessing any more."

He said maybe he can even help the Broncos do something they haven't managed to do yet.

"We're 0-3 in the preseason and even though that doesn't mean anything, to me as a competitor, I want to help this team win a game," he said.

NOTES: Rookie running back Knowshon Moreno took some practice repetitions for the first time since spraining his knee in the Aug. 14 preseason opener at San Francisco, but McDaniels said he's unsure whether he'll play Thursday. ... Running back Lamont Jordan resumed working out after suffering an unspecified leg injury two weeks ago. Wide receiver Jabar Gaffney (thumb) and guard Chris Kuper (ankle) also were among the injured players held out of practice. ... The Broncos released veteran tight end Jeb Putzier, rookie punter Britton Colquitt, wide receiver C.J. Jones and center Blake Schlueter, a seventh-round pick out of Texas Christian, to reach the 75-player limit.