J.T. O’Sullivan, quarterback for the 49ers, spent just under one month on the Patriots’ practice squad in 2006.

MCT / Jose Carlos Fajardo

SAN FRANCISCO — John Thomas “J.T.” O’Sullivan has a quarterback rating of 90.0, which ranks him 13th in the NFL through four games, and a persistency rating that’s off the charts.

Long after most guys would have quit pro football and joined the workaday world, O’Sullivan suddenly — and, probably to just about everyone outside his immediate family, shockingly — has earned the starting job as quarterback of the San Francisco 49ers.

The reason that’s so surprising is that O’Sullivan had been on the roster of seven NFL teams in six years before coming to the 49ers and had appeared in exactly eight games, seeing significant playing time in only one.

One of those teams was the Patriots, with whom O’Sullivan spent just under a month on the practice squad in 2006.

The Pats picked him up a few days after he’d been waived by the Vikings on the final cut at the end of training camp last month. New England let him go on Oct. 2, and O’Sullivan’s next in a series of stops was Carolina, which signed him to their practice squad that December.

Since then, he’s bounced to Chicago, and then Detroit, before landing in San Francisco.

“For us,” said Pats coach Bill Belichick, when asked why O’Sullivan had such a short stay in New England, “I don’t think it was a question of talent, or anything else. It was more of a question of opportunity, and reps, and how many quarterbacks you can work with.”

And what had the Patriots seen in O’Sullivan to sign him?

“I think what we’ve seen this year — athletic, good arm, live arm, can make all the throws,” Belichick said. “He has quick feet, can stay alive in the pocket, can improvise, make some plays scrambling around.”

How much longer O’Sullivan can stay alive — in the pocket, or out of it — with the 49ers is a legitimate question these days.

He was sacked six times last Sunday at New Orleans, bringing his season total to a bone-rattling, league-high 19 — including a punishing eight at the hands of the Seahawks.

“I’m looking to hang in there as long as I can,” he said, “and make the throws down the field.”

Clearly, O’Sullivan is a guy who knows how to hang on.

A sixth-round draft choice of the Saints in 2002 out of Cal-Davis, he never got into a game with New Orleans, which traded him to the Packers midway through the 2004 season. He actually got to take a snap at the end of the final game that season in Green Bay, but was waived the following September.

He next wound up in Chicago, on the practice squad, but was signed by the Vikings as a third-stringer in December. After that, he went to the Pats, then the Panthers, back to the Bears, and, in the summer of 2007, the Lions, where the offensive coordinator was Mike Martz.

Martz had been the creative genius behind the “Greatest Show on Turf” — aka the high-powered, wide-open offense of the NFC champion St. Louis Rams — in 2001. They were expected to be the NFL champion St. Louis Rams, but were outplayed — and outsmarted — by Belichick’s Patriots in Super Bowl XXXVI in New Orleans.

Things went downhill for Martz in St. Louis after that, and he eventually landed in Detroit.

Martz liked what he saw last season of O’Sullivan, who saw his first meaningful action in relief of Jon Kitna in a game against the Vikings, completing 13 of 24 passes for 148 yards and a touchdown, with two interceptions.

Fired last year by the Lions, Martz came to San Francisco to work for Mike Nolan and, with QB Alex Smith — the first overall pick in the 2005 draft — suffering from shoulder problems, suggested the Niners bring in O’Sullivan. Who, after years of being buried at the bottom of depth charts around the league, now is the starter.

He spent enough time in New England to know he’ll be challenged today by the Patriots.

“They’re going to give you multiple coverages and different looks, trying to confuse you,” O’Sullivan said. “But I’m not going to try to overanalyze what I’m seeing. I’m trying to recognize what I see, react to it, and take advantage of it.”

Belichick credits O’Sullivan for taking advantage of his opportunity with the Niners.

“He got into a good competitive situation in San Francisco,” Belichick said, “and made the most of it. I think he deserves credit for his perseverance and making that happen.”

jdonalds@projo.com