SANTA CLARA, Calif. — Just when J.T. O'Sullivan could use some extra work with the San Francisco 49ers' starting receivers, he's probably going to spend most of his final preseason game on the bench while the Niners figure out just who those starters will be.

O'Sullivan is unlikely to play much when the 49ers host the San Diego Chargers on Friday night, with coach Mike Nolan preferring to rest his new starting quarterback for the regular season. At least O'Sullivan will be joined in his brief work by receivers Bryant Johnson and Ashley Lelie, who will make their preseason debuts at Candlestick Park.

"It's going to be exciting to see those guys get in there and make plays," O'Sullivan said Wednesday. "Obviously (Lelie has) been out of practice every day, so this is a good start. We're starting to get something going there, so it's going to be exciting to see him play."

Lelie, a former University of Hawaii standout and the school's first first-round NFL draft pick, is on the bubble for a roster spot with the 49ers, while Johnson must fend off the excellent play of rookie Josh Morgan to keep a starting job in his first season with San Francisco. Johnson returned to practice Tuesday after missing three weeks with a hamstring injury, while Morgan will sit out against the Chargers to recover from an illness.

Lelie returned to practice Monday after missing most of training camp with a torn calf muscle. The 49ers' best deep threat is pretty much useless if he can't run at full speed, so the veteran receiver probably must play well against San Diego's defense to guarantee a roster spot.

"We haven't seen Bryant Johnson, nor have we seen Ashley Lelie for the entire preseason so far," Nolan said. "So I'll be anxious to see how they do in the game. ... It'll be a good problem if it's a tough decision, because it means that we have more than one guy. All of those guys will be called upon to contribute."

Nolan isn't sure whether Johnson or Lelie will start at split end, but both receivers are likely to play much of the first half. The coach structured his practices this week to pit the 49ers' offensive starters against the top defenders, providing an additional window of evaluation for the upcoming roster cuts.

O'Sullivan actually downplayed the importance of chemistry with his receivers, claiming, "everybody has to do things exactly the same. Things don't change for me depending on who's out there or what the situation is in the game, or who you're (playing) with. ... They trust me to put the ball exactly where I'm supposed to, and I trust them to be exactly where they're supposed to be."

Offensive coordinator Mike Martz sees it in milder terms, believing a quarterback's chemistry with his receivers plays a role in that execution. That's why Martz has kept O'Sullivan at work with Johnson and Lelie in practice this week, working on their timing and familiarity.

Whatever the circumstances, O'Sullivan is grateful to be preparing for the regular season instead of waiting for final cuts. With his well-traveled NFL history, O'Sullivan is no stranger to the knock on a hotel door or a cell phone call sending him away from yet another team.

"It's terrible," he said. "It's really indescribable, as far as I'm concerned. It's a range of emotions from embarrassing to disappointing. ... Everyone in the country knows you just got fired. It's put in the paper. I check the transactions. That's what happens. You get fired, it's embarrassing. That's how I look at it."