The third-year quarterback is closer to ready for that role than he's ever been.
Osweiler led four scoring drives in an efficient final tuneup for a third year behind Manning, and the Broncos beat the Cowboys 27-3 Thursday night, sending Dallas to the fifth winless preseason in franchise history.
Drafted by John Elway to be groomed as Manning's replacement, Osweiler had his best preseason yet, engineering eight scoring drives in four games and completing 29 of 49 passes for 416 yards with two touchdowns and two interceptions.
''Coach (John) Fox has a saying, `If you're not getting better, you're getting replaced,' so that sat with me pretty hard since my rookie year,'' Osweiler said after directing three drives that ended in rushing touchdowns. ''I was very happy with things, but this is a business where you consistently have to keep getting better, and that's what we're going to do.''
With Manning and Dallas' Tony Romo watching in uniform from the sidelines, Osweiler played three quarters, completing nine of 13 for 190 yards without an interception.
''He's matured so much in our system,'' Fox said. ''He's gotten those live games reps that are so critical in a quarterback's development. I think he showed pretty well again tonight.''
Romo's backup, Brandon Weeden, came out before halftime of his second ragged game in four preseason outings. He was 6 of 12 for 75 yards with an interception that set up Denver's first touchdown.
The Cowboys (0-4) didn't make the playoffs in three of their other four winless preseasons and are trying to avoid matching a club record with a fifth straight year of missing the postseason.
While Jerry Jones said he was concerned, he recalled his first season as owner in 1989 when he had fired coach Tom Landry. Dallas was unbeaten going into a preseason game against Denver and Broncos coach Dan Reeves, who played for Landry.
''He was hot about coach Landry and those things that had been done here with the Cowboys,'' Jones said. ''He left Elway all the way through the whole game and we played into overtime ... so we didn't go undefeated. And we won one football game the whole season. So much for the preseason.''
Kapri Bibbs, a running back on the roster bubble, rushed for two second-half scores for the Broncos (3-1).
''I think this was a game that was very important for young guys like me to go out there and give it your all and leave it all out there on the line,'' said Bibbs, who averaged 4.6 yards on seven carries. ''Make the decisions for the coaches even harder and make the other NFL coaches take a look. It was real good.''
The Broncos shared most of their running plays between running backs expected to make the team in C.J. Anderson and Juwan Thompson. The Cowboys sat their top four runners and handed the ball to guys who might not be around after final cuts Saturday.
Anderson got the heaviest load with 14 carries for 33 yards and a 1-yard touchdown set up by Osweiler's 34-yard pass to a leaping Andre Caldwell near the goal line.
One of the Dallas backs was Phillip Tanner, who wasn't re-signed by the Cowboys after spending his first three years with them. Tanner was cut by the Colts last week, so the Cowboys officially added him earlier Thursday just to get through the game. He led Dallas with 26 yards.
As expected, Denver defensive end DeMarcus Ware didn't play in his first trip to Dallas since the Cowboys' franchise sacks leader was released and signed with the Broncos in March.
Ware had a long pregame chat on the field with Jason Witten, the Dallas tight end who interrupted Ware's conference call with reporters this week for some playful trash talk.
Weeden led a 74-yard drive to a field goal, but was off the mark on several throws and had just one first down in three other possessions.
Several Dallas defensive starters got a few snaps, but Henry Melton wasn't one of them.
The Cowboys changed their plan to get their free agent tackle his first preseason action before the opener, taking the safe route instead with his groin injury. Melton hasn't played since a torn knee ligament ended his final season in Chicago in Week 3 last year.
NOTES: To make room for Tanner, Dallas waived CB B.W. Webb, a fourth-round pick last year. ... The Broncos sacked third-string Dallas QB Dustin Vaughan four times. ... New Denver K Brandon McManus was 2 of 4 with misses from 52 and 54 yards.
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