SAN DIEGO — Everything you need to know about last night’s mismatch between the San Diego Chargers and the New England Patriots of San Jose took place on each team’s first possession of the third quarter.

The Pats took the second-half kickoff and drove to the San Diego 1. Four plays later, they were on the 2. It then took the Chargers just five plays to drive 98 yards for a touchdown that gave them a commanding 24-3 lead.

It was struggling San Diego — 2-3 coming into the game after a loss last week at Miami (which lost yesterday to previously winless Houston) — that looked like the defending AFC champion.

The Patriots looked like…like…well, truth be told, they looked like San Jose State, where they spent the last week practicing — not that you could tell they’d been practicing from the way they played last night.

The first half was bad enough as the Chargers racked up 216 yards in total offense on their way to a 17-3 lead while the Pats, struggling without NFL MVP Tom Brady at quarterback, managed a mere 87.

Then, in the second half, it got worse.

A lot worse.

Although it didn’t seem that way at the outset, as the Pats appeared to be on their way to cutting their deficit to a touchdown after driving from their own 22 to the Chargers’ 1.

But what seemed encouraging quickly became embarrassing as the Patriots not only failed to get into the end zone, but wound up losing a yard.

Matt Cassel, coming up short in the unenviable — and, in fairness, all but impossible — task of filling Brady’s cleats, threw incomplete on first down. The Patriots tried to run on second down, but defensive lineman Luis Castillo bulled into the backfield and dropped Sammy Morris at the 2.

Cassel again threw incomplete on third down, then found himself scrambling on fourth down after being unable to find an open receiver. He never got near the goal line, instead getting smothered by tacklers at the 2.

Inspired by that defensive stand, the San Diego offense, under the direction of the highly capable Philip Rivers, quickly zipped down the field for a touchdown that left the Pats red-faced and reeling.

The big play was a 59-yard pass to Vincent Jackson. It was hardly the first time the New England secondary had been burned deep by Rivers. Indeed, on the first play of the game, Rivers hit Jackson for 48 yards, going right at cornerback Deltha O’Neal, who allowed Jackson to run past him, then couldn’t catch up.

Encouraged by that bit of early, and easy, success, Rivers went after O’Neal again on San Diego’s next possession and found Michael Floyd — listed third on the Chargers’ depth chart at wide receiver, behind the injured Chris Chambers and the unheralded Buster Davis — for 49 yards and a touchdown.

So it was the proverbial “team loss” for New England.

The offense couldn’t score, and the defense couldn’t stop the Chargers from scoring.

It’s not likely to be the last time that happens in what is shaping up as a frustrating season for Pats fans spoiled by success, highlighted by last year’s 16-0 regular-season record.

With Brady throwing to Randy Moss and Wes Welker, the Patriots set an NFL record last season by scoring 589 points. This season, with the exception of last Sunday’s 30-21 win over the 49ers in San Francisco, the New England offense hasn’t scored more than 19 points in a game.

Not that that should be surprising, considering that, prior to starting the second game of this season, Cassel hadn’t started a game since high school.

Too often last night, the Chargers made the Pats look like a high school team, instead of the perennial playoff team they have been since 2001, when Brady replaced Drew Bledsoe.

Fans hoping for a similar miracle — that Cassel could step in for Brady the way Tom Terrific did for Drew — will have to learn to be patient while Cassel learns how to run an NFL offense.

That patience certainly was not in evidence the last time the Patriots played in Gillette Stadium.

Fans were pouring out of the park early in the second half as the Pats, who had won a league-record 21 regular-season games in a row, were routed by the lowly Dolphins — losers of 20 of their previous 21 games — 38-13.

It’ll be interesting to see how they — and the Patriots — respond next Monday, when the Broncos come to Foxboro for another prime-time game.

Last night, unfortunately, the Pats looked like the Not Ready for Prime Time Players.

jdonalds@projo.com