Kansas City, Mo. ? The box score at halftime of the Chiefs’ loss to San Diego last weekend resembled that of a high school offense running the wishbone.

Certainly when it came to the passing statistics.

Matt Cassel completed all but one of his attempts, but they went for a whopping 18 yards. The longest completion by the Pro Bowl quarterback went for just four. And by the time Kansas City trotted off the field in yet another 10-0 hole, fans were yelling in unison: Throw it deep!

Well, someone must have heard them. Kansas City opened up the playbook in the second half, Cassel wound up throwing for 176 yards and two TDs, and the Chiefs’ offense started humming for the first time all season, nearly coming all the way back for a much-needed victory.

“In the second half, it just started to roll a bit,” Cassel said. “We started to hit third downs, and you could see the confidence in the offense starting to rise. It was an impressive performance, I’d say, in the second half, something to build on.”

Cassel has taken much of the criticism for the Chiefs’ winless start, and for good reason. He’s thrown for just 428 yards with three touchdowns and five interceptions.

To put that in perspective, Cassel is on pace to throw for 2,282 yards with 16 touchdowns and 27 interceptions this season, for a quarterback rating of just 65.5. Not even Brodie Croyle put up such dismal numbers in Kansas City, and the much-maligned Todd Blackledge managed more yards-per-completion than Cassel’s 7.9 during his checkered career at Arrowhead Stadium.

All despite the fact that teams across the league have averaged more yards passing and TDs through the air than the first three weeks of any other season in NFL history.

“There’s disappointment any time you play the quarterback position in the NFL. You put yourself in that position,” Cassel said. “You have to learn to turn the page.”

Not all the blame rests on Cassel’s shoulders.

Offensive coordinator Charlie Weis left after last season to take over the same position at the University of Florida, and coach Todd Haley promoted longtime assistant Bill Muir to the job. Despite decades of coaching experience, Muir has never before called the shots, and Haley’s background on that side of the ball has cast uneasiness over who’s really in charge of the entire offense.

Many assumed that Haley had taken over calling plays at halftime of the Chargers game, particularly given how Kansas City opened up the second half. But that notion was quickly shot down by Haley, who said play-calling has been and will remain a collaborative effort.

“Bill Muir is our play caller,” Haley said. “I thought he did a tremendous job of just sticking with the plan. We had a clear-cut plan, we felt really strong as a staff in all areas, but offense was one of those areas that we couldn’t get out of the plan too early.”

The Chiefs eventually did get out of the plan, or at the very least changed it up.

On third-and-6 in the first quarter, the Chiefs ran a short pass play to Dexter McCluster that gained just three yards. On their ensuing possession, facing third-and-4, a pass play to Jackie Battle again gained three yards. And on the Chiefs’ third chance with the ball, again facing third-and-4, Cassel hit McCluster for three more yards and Kansas City was forced to punt.

Things changed on the Chiefs’ first possession of the second half.

Cassel hit tight end Leonard Pope for eight yards, missed on a deep throw to Steve Breaston — but at least took a shot down field — and finally connected with Dwayne Bowe for 23 yards.

That set up a short touchdown throw that got the Chiefs within 10-7, and the Chiefs continued to air it out until their final offensive play.

Trying to get into range for a tying field goal in the final minutes, the coaching staff reverted to first-half form and called a screen play. It was picked off by a blitzing Eric Weddle, and the Chargers ran out the clock to preserve a 20-17 victory.

“We weren’t able to run the ball enough, we weren’t able to throw the ball enough, wherever we were throwing it,” Haley said. “But that again doesn’t fall on any one person’s shoulders. Everyone has to have a little better understanding of what they have to do and how we want them to do it.

“I thought offensively, to start the second half, the offense came out and despite feeling pretty bad about themselves went down the field and got points,” Haley added. “I thought our entire team continued to fight and make the outcome what we wanted it to be.”