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By ARNIE STAPLETON

AP Pro Football Writer

ENGLEWOOD, Colo. (AP) - That's what 18 months as No. 18's understudy looks like.

Brock Osweiler followed Peyton Manning's cameo appearance Thursday night with his most productive day in a Broncos uniform in Denver's 10-6 preseason win at San Francisco.

Although he didn't produce any points as the Broncos' battered offensive line had its hands full with the 49ers' relentless pressure, the second-year pro displayed his accuracy and athleticism in completing 13 of 18 passes for 105 yards.

After completing just two passes as a rookie, Osweiler has looked much more comfortable in running Denver's offense this summer, helping the Broncos breathe easier and giving Manning a high-quality backup, the likes of which he never really had during his time in Indianapolis.

While it was pretty much accepted last year that all bets were off if Manning's neck issues resurfaced or he otherwise got hurt, there's no longer a foreboding sense that there would be a huge drop-off if Osweiler is pressed into duty.

The second-year pro looks nothing like the wide-eyed rookie he was a year ago when he began his apprenticeship under Manning with his head spinning, his mechanics needing tweaking and his selection in the second round of the NFL draft by a team with championship aspirations being hotly debated by a fan base eager for immediate help.

He's decoding defenses, threading throws over defenders, letting plays develop downfield.

A lot like Manning, actually.

Yeah, that is a tribute to his work ethic,'' Broncos coach John Fox said.He has got a pretty good teacher there at 18 and he has handled that very well. He has grown from it and I think he just continues to improve.''

Osweiler said another year under his belt has him feeling much better under center.

Last year, things were coming at me a million miles per hour,'' he said.Things have really seemed to slow down. I think a large credit to that is just the fact of having an offseason. In that offseason, I was given time to study, look into the playbook further. Why are we calling this pass play? Why are we calling this run play? What does this protection actually do? Now things have slowed down to the point where I'm able to just go out there and play and not think as much.''

Because his head's no longer spinning, ``I'm able to make more plays downfield, which I wasn't really ready for last year. It was a lot of check-downs, quick throws, because I didn't understand the offense completely,'' Osweiler acknowledged.

Broncos boss John Elway took a lot of heat for selecting Osweiler in the second round out of Arizona State last year rather than drafting, say, running back Doug Martin, who was snatched up by the Tampa Bay Buccaneers and had a Pro Bowl season.

The Broncos sure could have used a bruising running back in the playoffs, when they couldn't run out the clock and were upset by the Baltimore Ravens in double overtime.

Elway, though, subscribes to the philosophies of former Green Bay general manager Ron Wolf, who drafted a quarterback almost every year despite having Brett Favre under center, and he insists his selection of Osweiler will pay dividends for years to come.

Osweiler might have been the first quarterback taken in this year's draft had he stuck around for his senior year at Arizona State, but he figures learning from the best in the business is better for his football future anyway.

And he swears he never laments landing in Denver, where there's already a four-time MVP under center, instead of another city where he might already be the starting quarterback.

``He hasn't come to me complaining that he thinks he's better than Peyton at this point,'' Fox cracked last week.

Osweiler insists he's happy to bide his time.

He replaced Manning on Thursday night after one drive and completed his first six passes. His best was an 11-yard rope to well-covered tight end Julius Thomas. His best drive came when he took the Broncos from their own 2 to the Niners' 7, where rookie running back C.J. Anderson was stopped just short on a fourth-and-inches plunge.

Brock has really worked hard this offseason,'' Manning said.I think he's really learned a lot from (quarterbacks) Coach (Greg) Knapp, who's been around the block quite a bit. He's put in a lot of time on his own and I know he enjoyed playing tonight. He didn't get to play as much last year in the preseason, so I know he enjoyed the amount of reps that he got, and I thought he really did a good job.''

Manning figures to play plenty at Seattle on Aug. 17, the Broncos' last preseason road game and Manning's last chance before the regular season to work in the deafening din of a road stadium with his new teammates Wes Welker and Louis Vasquez along with new center Manny Ramirez and new tight end Thomas.

``The more this offense can play together, I think the better we can be,'' Manning said.

Osweiler will also get plenty more snaps in the preseason.

If there's one thing he's learned above all else from Manning, it's to prepare as though he'll be the one starting.

Because one day he will be.

People say I'm the future, I'm the heir apparent. Those are only words,'' Osweiler said.I have to go out there, I have to earn that right.''

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