SANTA CLARA, Calif. — If Alex Smith can't reclaim the San Francisco 49ers' starting quarterback job over the next 17 weeks, he can forget about getting another chance next season.

General manager Scot McCloughan confirmed the harsh realities of NFL salary cap life today for Smith, the former No. 1 overall draft pick who will begin his fourth pro season Sunday as the backup to journeyman J.T. O'Sullivan. Although the 49ers appear committed to keeping Smith this season, given the meager benefits of cutting him and the minuscule chances of trading him, their largesse won't continue in 2009.

When asked if the 49ers could keep Smith as anything less than the clear-cut starter next year, McCloughan said: "You can't."

"The amount of money we're going to invest in him, he'd have to be proven that he's the guy," McCloughan added. "When we get to the offseason, he'd have to be the guy for next year."

If the 49ers' decision to start O'Sullivan works out, Smith could make a hefty salary just by holding a clipboard this season. Smith also is scheduled to make more than $9 million in base salary next year, with a cap figure over $12 million.

McCloughan acknowledges he was "surprised" last month when O'Sullivan beat out Smith, who was McCloughan's first draft pick with the 49ers three years ago. McCloughan expected Smith to be the Niners' starter Sunday against Arizona, getting the experience and seasoning he would need to be the long-term starter.

But O'Sullivan, the eight-team NFL veteran who spent last year in Detroit with 49ers offensive coordinator Mike Martz, apparently has a more thorough knowledge of San Francisco's new offense than Smith, who's on his fourth offensive coordinator in four years. O'Sullivan will make his first NFL start against the Cardinals.

"Am I disappointed? Not whatsoever," McCloughan claimed. "I think that you would have gone into this last offseason thinking that our starting quarterback would be (found) on the street. I don't think you'd find one team that would say that. ... We brought a quarterback in that fits the system that we're going to run, and he showed during the offseason and preseason that he was getting better with the system and it was working for him."

Smith is well aware of his predicament and his imminent departure from San Francisco unless he gets the job back. His tense relationship with Nolan in 2007 has largely cooled, and Smith remains publicly positive about his chance to contribute in San Francisco this season.

"I'm ready to do whatever they ask me to do," Smith said recently. "My shoulder feels fine. (I'm) just looking forward to learning all of the offense and playing whatever role I get."

In the first few months since McCloughan's promotion to a nominal position of authority over coach Mike Nolan, the GM has stayed out of coaching decisions such as O'Sullivan's promotion, just as he did over the past three seasons when Nolan had the final say on every decision. McCloughan claims he and Nolan still make personnel decisions together, and they almost never disagree.

The general manager doesn't regret giving $24 million in guaranteed money to Smith despite his middling statistics with one of the NFL's worst offenses over the last three years. McCloughan also insists Smith had a fair chance to win the starting job, contrary to rumors that O'Sullivan got the nod despite doing little to distinguish himself in practice.

"From a pure talent standpoint, I see something different, but that doesn't matter," McCloughan said. "You've got to go out there with the guy who understands the system and knows how to work the system best."

Nolan managed to get a fourth year at the 49ers' helm from owner John York despite a 16-32 record in his first three seasons. Now that McCloughan has the authority to fire the coach who hired him to run the personnel department in 2005, he's making no bold predictions about Nolan's future beyond this season.

"We're all under fire, from the standpoint that we need to win," McCloughan said. "We have not won as many games as we would have liked so far. Not making excuses, we feel this team is talented enough to win. We've got to go out and do it. I wouldn't pinpoint one guy that's under more pressure than another guy. We all work together."