In the aftermath of the 2007 season, the departure of J.T. O'Sullivan was merely a footnote.

O'Sullivan had won the Lions' backup quarterback job a few months before. But his biggest supporter, offensive coordinator Mike Martz, had been replaced.

The Lions had their guys — veteran Jon Kitna, plus draft picks Dan Orlovsky and Drew Stanton. O'Sullivan was on his seventh NFL team. His one-year contract was up.

Though the Lions spoke to O'Sullivan's agent, they let him go in free agency without a word.

"I didn't have any personal conversations with them, no," O'Sullivan said Wednesday.

Now O'Sullivan is San Francisco's starting quarterback, and the Lions — reeling from a 0-2 start — visit the 49ers on Sunday.

The perception is that O'Sullivan was Martz's guy. But O'Sullivan said he had a good relationship with the other coaches in Detroit and didn't assume he would end up in San Francisco after Martz joined the 49ers.

"It was just like every other opportunity," O'Sullivan said. "You look for the best opportunity to first make a team and then have a chance to compete, and that's the way it was expressed to me. This was the best opportunity for that."

It might have been his only opportunity. O'Sullivan declined to discuss the options he had.

Regardless, it's amazing, considering the 49ers drafted a quarterback first overall in 2005, Alex Smith, and gave him a $49.5-million contract that included $24 million guaranteed.

Smith indirectly led to O'Sullivan's arrival. San Francisco coach Mike Nolan wanted someone to work with Smith, and he and Martz had been on staff together with Washington.

When Martz became available, Nolan hired him. Not long afterward, O'Sullivan followed.

"I needed somebody to coach a quarterback, seeing as I had a young guy," Nolan said. "Anyway, we wanted to add a third player to the roster to play quarterback. We didn't want to get somebody that wouldn't compete. We wanted to get somebody we thought could at least compete for the position."

O'Sullivan joined his eighth NFL team, the 49ers, whose training camp he attended growing up in California. He signed a one-year deal for the veteran minimum of $645,000.

He had grasped Martz's complex offense quickly in Detroit and had a season to study it. In the off-season, he got few reps behind Smith and Shaun Hill. But when he got a chance, he played well. The first five days of training camp, he got zero reps. But same thing — when he got a chance, he played well.

"It's been a long enough process for me," O'Sullivan said. "I've gone through good ways to do it and bad ways to do it, and I just think the best way to do it is to try to play the position exactly as you're being coached, not try to do too much. ... It's just a matter of getting an opportunity to do it."

O'Sullivan started the exhibition opener and won the job after the third exhibition.

In two games, a loss to Arizona and an overtime victory at Seattle, O'Sullivan has gone 34-for-52 for 516 yards — one touchdown, one interception, 12 sacks. His passer rating is 96.3.

In two games, both lopsided losses, Lions quarterback Jon Kitna has gone 45-for-74 for 538 yards — four touchdowns, four interceptions, eight sacks. His passer rating is 78.5.

"He's one of the best throwers of the football I've ever been around," Kitna said. "He's a really good quarterback. He can throw from any angle. It doesn't matter if he's off-balance, whatever, he gets it out quick."

The Lions' coaches and players praise O'Sullivan's intelligence, release and mobility. But why did the Lions let him go then?

"He hasn't played in this league long, so he'll make his mistakes," wide receiver Roy Williams said. "Hopefully we can confuse him, and I teach these guys some things about what they're going to see, and we can take some picks to the house."

O'Sullivan said he doesn't need any extra motivation to face the Lions. He has been scratching and clawing to make it in the NFL for years and now, finally, this is his chance. This is his third NFL start. That should be motivation enough.

"I think guys like that always have something to prove," Lions coach Rod Marinelli said. "And he's doing a great job of proving it."