ENGLEWOOD, Colo. - The Denver Broncos are searching for someone to watch Peyton Manning's back after left tackle Ryan Clady tore his left ACL and will likely be out for the season.

It's a big responsibility and tryouts begin immediately.

The requirements are simple: Don't let anyone hit the five-time NFL MVP — something Clady was deftly able to do.

Clady hurt his knee during a voluntary practice Wednesday, an injury that no one thought was really all that serious. Clady was setting up to pass block when his knee buckled, Broncos coach Gary Kubiak said. He even took another snap before pulling himself out of practice because his knee was sore.

"We really weren't that worried last night. Everybody felt good," Kubiak said. "Thought we'd take a look at it (Thursday). Unfortunately, we got some horrible news."

And just like that, Clady was gone.

For how long, though, Kubiak wasn't willing to speculate, although his return in 2015 appears improbable.

With Clady gone for the foreseeable future, the Broncos will now be relying on four new linemen to keep Manning safe — five if you count Louis Vasquez, who's moving from right tackle back to his more comfortable position of right guard.

"It's our job to protect Peyton. If we're not doing our job, he can't do his," Vasquez said. "There's no room for error up front."

The work on revamping that offensive line begins immediately because, "They're not going to stop the game or give us a mulligan or nothing. We're expecting to play," offensive co-ordinator Rick Dennison said.

"Certainly, we have a lot of talent," Dennison added. "We're going to win some games."

This isn't the first time Clady has blown out his left knee: He hurt it during the off-season in April 2010, only to return and still start every game that season.

Clady also injured his left foot in the second week of 2013 and missed the rest of the season, not long after signing a five-year, $52.5 million contract. He returned last season and started every game. He made his fourth Pro Bowl squad as he and his fellow linemen surrendered an NFL-low 17 sacks.

"We were counting on Ryan," Kubiak said. "That's the challenge of this business."

On Thursday at practice, shortly after finding out about Clady's injury, the team inserted rookie Ty Sambrailo into his spot. The second-round pick played quite a bit of left tackle at Colorado State.

"He's not a rookie anymore," Kubiak said. "As of today, he's about a four-year player. We've got to get him going."

Denver also will give Michael Schofield a look along with versatile veteran Chris Clark, who filled in for Clady in 2013. But Kubiak prefers to keep Clark at right tackle, where he's been lining up throughout the off-season training activities.

For now, the rest of the line looks like this: converted defensive lineman Ben Garland at left guard and Gino Gradkowski at centre.

"Ryan is obviously not only a veteran player, but a leader on this team, too. It makes it even more difficult," Kubiak said. "But if you're in this business — playing, coaching — and been around for a while, you're going to face very difficult things like this. How you respond has a lot to do with what your football team ends up being.

"It's a tough road, but it's one you have to step up and meet the challenge. It's our job to step up and get that done."

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