COSTA MESA, Calif. (AP) — Kellen Clemens has learned plenty about preparing and not playing. After all, he has been the backup to Brett Favre and Philip Rivers, two of the most durable quarterbacks in NFL history.

That's why Clemens believes he could step in smoothly for his first significant NFL action in nearly four years if Rivers doesn't start for the Los Angeles Chargers on Sunday for the first time since 2005.

Clemens and the Chargers are increasingly confident they won't have to find out, however.

Rivers practiced with the Chargers on Wednesday while progressing through the league's concussion protocol, and coach Anthony Lynn is optimistic that the veteran quarterback will play at home against the Buffalo Bills.

"I just let the doctors handle that, and Philip will be brutally honest with us," Lynn said. "I told him, 'Be honest with us about this injury,' and he has been so far. That's all we can go by."

Rivers has started 194 consecutive games for the Chargers since the 2006 season opener, racking up the fourth-longest streak of consecutive starts by a quarterback in NFL history. Favre holds the longest streak at a jaw-dropping 321 games between 1992 and 2010.

But Rivers was hurt at some point during an overtime loss in Jacksonville last weekend, and he reported the symptoms to the team on Monday before entering the concussion protocol.

Rivers participated in certain parts of a non-padded practice Wednesday, and Lynn plans to increase his workload during the week if he feels good. The Chargers will continue to monitor his health until Sunday morning, but early signs are encouraging.

If Rivers can't play, Clemens is expected to make his first NFL start since 2013 with the St. Louis Rams. Clemens must practice all week without knowing whether he'll be responsible for every snap or none against the Bills.

"That's every week for the majority of my career," he said with a smile. "I'm used to it. ... Everybody hopes that Philip is going to be out there and ready to go. But if called upon, I'll be ready to go."

Clemens has thrown only 10 passes in the past 3½ years while serving as Rivers' backup in San Diego and LA, but the veteran has done enough in practice to be confident — and for the Chargers to be confident in him.

"Kellen is very involved with the game plan part of it," Chargers offensive coordinator Ken Whisenhunt said.

"He's been in the system for a number of years, so he's comfortable with it. You don't want to make him try to do too much. Maybe you cut it down a little. But from all the guys I've been around, especially in that role, Kellen is one of the sharpest as far as understanding what you need done and communicating that to the players.

"Executing it in a game is something he hasn't done a lot of in the last couple of years, but he's a pro, and I'm sure he'll be prepared."

Clemens is in his 12th NFL season with his fifth franchise. He began his career with the New York Jets, backing up Chad Pennington, Favre and Mark Sanchez while getting sporadic playing time, including eight starts in 2007.

Clemens started nine games for the Rams and passed for a career-high 1,673 yards in 2013 after Sam Bradford was injured. He has barely seen the field on Sundays since he signed with the Chargers in 2014, but the 34-year-old Oregon product has taken an increasingly large role in game-week preparation and game-planning.

"Your first couple years in the league it's, 'Hey, rookie, sit over there and shut up and just watch,'" Clemens said. "But as you get older, you gain some respect. It's something that coaches are a little more open to. I've been fortunate with Coach Whiz and (quarterbacks coach) Shane (Steichen) as well as Philip. If I have an idea, then they listen. It doesn't always make it in the game plan, but at least they listen."

Clemens and Rivers are friends, and Clemens credits the 10th-leading passer in NFL history for his openness to collaboration.

"He's an open book," Clemens said. "A lot of guys aren't, and they don't have to be. But he's an open book. If I have a question, he takes the time to answer it. If I don't understand it, he explains it a different way. It's a relationship I'm very grateful for, and I've benefited from in ways that haven't been seen yet."

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