FOXBORO –– The New England Patriots’ defensive players should have one mindset when it comes to Sunday’s game against the San Francisco 49ers –– pressure, pressure and more pressure.

While defensive leader Richard Seymour said yesterday the Pats’ low number of sacks this season is in part due to its struggles against the run, he and his teammates have to be watching video of the 49ers with eyes wide, thinking of the havoc they could wreak this weekend.

San Francisco offensive coordinator Mike Martz’s schemes are complex and pass-heavy. That worked fine when Martz was the coach in St. Louis back in the Greatest Show on Turf Days, with Kurt Warner under center, and Marshall Faulk at running back, and Isaac Bruce and Torry Holt at receiver.

But Martz is working with J.T. O’Sullivan at quarterback, and his inexperience coupled with the complexities of and high number of long passes in the playbook have led to some protection problems for the Niners.

O’Sullivan, at 29 and in his first season as a starter in fact, his first season where he’s getting a regular number of snaps, has been sacked 19 times in four games, nearly five takedowns per contest. He’s on pace to be sacked 76 times this year.

Pressure, pressure and more pressure.

No one is better than building up an opponent than Bill Belichick, and yesterday he said San Francisco’s offensive line has held up well thus far.

“They throw a lot of downfield routes; they throw deep in-cuts, deep comebacks,” Belichick said. “Sometimes they have a lot of receivers out, so occasionally their protection breaks down on that. But they attack the defense at all three levels: the short, intermediate and deep (passing) levels. They do it every week on a consistent basis, so you have to be ready to defend it. Sometimes you get to [the quarterback] before he gets it off, but a lot of times you don’t and they hit a lot of 20-, 25-, 30-yard completions. A lot more than most teams do.

“It takes a little longer for those plays to develop. I think overall their pass protection has been good. Like any line it has broken down from time to time but they have done a good job.”

Belichick added that there have been times when O’Sullivan has held the ball, others when the protection has broken down, and still others when the defense has blitzed successfully.

The New Orleans Saints sacked O’Sullivan six times on Sunday, and he fumbled once and threw two momentum-killing end-zone interceptions. In Week Two, against Seattle, O’Sullivan was sacked eight times.

Pressure, pressure and more pressure.

Though O’Sullivan is pretty green, Martz hasn’t changed his plays much.

“It’s Mike’s offense. It’s basically the same plays. He tailors (his offense) to the personnel he has,” Belichick said. “They shift. They motion. They run their passing concepts. They have a lot of formations, different personnel groupings. They run the same plays, but they are all different because it’s different personnel, different formations and different looks. That’s still what they do but they change things up a lot.”

Despite the issues at times, he noted, O’Sullivan still has a 300-yard passing game (against the Seahawks), and San Francisco can move the ball well. Running back Frank Gore, who has a combined 525 yards rushing and receiving, likely has a lot to do with that.

Seymour and Rodney Harrison said Gore reminds them of San Diego’s LaDainian Tomlinson in that he rarely comes off the field. The Patriots will play Tomlinson and the Chargers Oct. 12.

Harrison said New England needs to get back to being aggressive and physical to disrupt the 49ers’ receivers and put pressure on O’Sullivan.

smanza@projo.com