Last Chargers exhibition game is critical to the back half of the roster

No one can see it but everyone inside the Chargers’ new locker rooms in Costa Mesa knows it’s there — a giant ax ready to swing down and chop the roster nearly in half.

With the overwhelming majority of first-unit players sidelined for the last game of the preseason Thursday at San Francisco, it’ll be a final chance for players on the bubble to stay on the roster for at least another week.

Come this weekend, the Charger roster can only hold 53 names, and it would be impossible not to think about that if you were unsure of your future.

That kind of pressure can be overwhelming if you let it.

Undrafted free agent cornerback Michael Davis has impressed coaches with his physical tools, but a poor performance in the team’s first preseason game put a real dent in any momentum he built toward a roster spot this summer.

Davis has been more reliable since the Seattle game but he knows he’s on shaky ground.

“The whole idea of making the team or not, I think about it sometimes,” Davis said. “I think about it during practice, but usually around the middle of the week, it comes down to me just playing ball, having fun. The closer to game it just comes down to just playing and using what the coaches teach us in the classroom and using it on the field.

“You just have to play ball and see what happens.”

It’s advice former undrafted free agent Trevor Williams made sure to share with Davis.

Williams made his way to the active roster after being among the final cuts. He signed on to the Chargers’ practice squad and after an injury to Jason Verrett, quickly found himself in a key role.

“I know for me, last year, I wasn’t sure if I was going to cut or stick around. I wasn’t sure,” Williams said. “I just tried to control what I could control — that’s my play on the field — and whatever happens after that is in God’s hands. I try to tell the younger guys to just focus on your assignments, focus on your job and just try to put good things on tape. Even though it might not work out here, it could work for the other teams.”

The Chargers will have to make a tough decision at cornerback, which could come down to Davis’ ceiling or former third-round pick Craig Mager’s experience.

The team will also be evaluating what to do at safety, where it seems like only Jahleel Addae has a starting spot. Tre Boston and veteran Dwight Lowery are “neck and neck” for the other starting spot, according to coach Anthony Lynn. Behind them, rookie Rayshawn Jenkins, fourth-year veteran Adrian Phillips and journeyman Dexter McCoil all are in the running for roster spots.

“I think every game, every day you’re evaluated,” McCoil said of the final preseason game. “I look at it as the next game to go out and perform and do the best you can to help the team win.”

The team’s quarterback picture behind Philip Rivers still remains cloudy, with Lynn not committing to either Kellen Clemens and Cardale Jones.

The team could decide to keep all three quarterbacks, but like every decision, an excess at one spot creates a shortage elsewhere. The Chargers also are trying to figure out who will fill their backfield rotation behind Melvin Gordon and, presumably Branden Oliver, with Andre Williams, Kenjon Barner and Austin Ekeler trying to separate from one another. And with Max Tuerk’s four-game suspension looming, there’s suddenly an extra spot open on the offensive line.

When it comes to making the roster decisions, Lynn said there’s a checklist that he’ll work his way through.

“Does he know what he’s doing after our training camp? Is he consistent, dependable, in shape? In good enough shape to finish a ballgame?” Lynn said. “Then, we ask if he has enough talent.

“I kind of like it in that order. I’ve been around the league long enough to know it takes more than talent to win a championship.”

Thursday will be the last chance for guys such as Davis to prove that they’ve learned from mistakes, that they’re ready to be one of 53 on the roster to open the season.

“For the guys on the bubble, if they perform well, it’s going to weigh in pretty good for them,” Lynn said. “It’s a big game for a lot of players.”

dan.woike@latimes.com

Twitter: @DanWoikeSports