ENGLEWOOD, Colo. (AP) — Peyton Manning isn't expected to play in the Denver Broncos' preseason opener at Seattle on Friday night.

Coach Gary Kubiak has been managing Manning's snaps at training camp, giving the 39-year-old quarterback every few days off to keep his legs and mind fresh.

Manning faded down the stretch last season when he was hobbled by a strained right thigh. He looks fitter, trimmer and more mobile this summer as he adapts to a new offense that has him doing roll-outs and bootlegs and this week said, "I do feel good moving around out there, arm feels good."

Another benefit of keeping Manning's snaps limited is getting a look at backup Brock Osweiler.

Entering his fourth season of his prolonged apprenticeship — and the last year of his rookie contract — Osweiler is getting his biggest chance yet to prove he's a worthy successor to Manning.

Osweiler has shown flashes of solid play but still some mistakes on days that Manning watches practice in a golf cap.

"I just think he's gaining a little confidence running the football team when it's his," Kubiak said last weekend. "You can see when he comes out here on those days it's not a shock. It's, 'Let's go.' I think it's important for the group, and obviously, it's important for his career."

Kubiak's philosophy has always been to serve up more playing time to youngsters and backups in the preseason, and these games will present Osweiler with his truest tests.

"Real, live game reps," Osweiler said, "there's nothing that can replace those."

Osweiler's mobility and strong arm seem to fit Kubiak's offense well.

"When he steps in, we see no downfall," receiver Bubba Caldwell said. "We expect the same out of him as we do Peyton Manning. He came out of here and made plays, showing that if time presents itself, he'll be a game-changer for us."

Osweiler won't exactly be working with the first-team offense in Seattle, however, because Pro Bowl receivers Demaryius Thomas and Emmanuel Sanders also are expected to sit this one out along with new tight end Owen Daniels.

Thomas missed the offseason program before agreeing to a five-year, $70 million contract last month and is still getting into football shape and Sanders is coming off a strained hamstring.

Right guard Louis Vasquez has been getting "veterans' days off" but he'll line up with the young O-line that includes three players who have zero NFL starts.

The Broncos had an NFL-high 11 Pro Bowlers last season, but Ryan Clady (knee) is out for the season and Julius Thomas bolted for Jacksonville in free agency.

Of the nine left on the roster, only Von Miller, C.J. Anderson, T.J. Ward and Aqib Talib are expected to get some playing time against the Seahawks.

Cornerback Chris Harris Jr. has been bothered by a hamstring injury and is expected to be in street clothes alongside DeMarcus Ware and linebackers Danny Trevathan and Brandon Marshall. Trevathan and Marshall are working their way back following long rehabilitations from surgeries.