Kansas City, Mo. ? A game they absolutely, positively cannot afford to lose wasn’t supposed to come this quickly.

Yet, quarterback Trent Green is sidelined with head trauma. The offensive line is in turmoil. The entire offensive approach is in flux, and the defense is showing progress, but still working to adapt to a new way of going about the job.

The Kansas City Chiefs under Herm Edwards are 0-2, so expect a fired-up home team when the San Francisco 49ers and rapidly improving second-year quarterback Alex Smith hit town today.

An 0-3 start, just their second since 1980, likely would doom the Chiefs to a 13th consecutive year without a postseason victory. Coming off an encouraging 10-6 campaign, they were hoping for better.

“We can’t afford to be 0-3,” said wide receiver Eddie Kennison. “This is a must-win for us. I think everybody in the locker room will probably agree with that.”

Damon Huard will be the first quarterback whose name is not Trent Green to start a home game for KC since Elvis Grbac left the building in 2000. Green, injured in the opener Sept. 10, is out indefinitely.

Huard, who was Mr. Conservative while hardly attempting a downfield pass during in a 9-6 overtime loss at Denver two weeks ago, will be making his second consecutive start, and his second since 2000. The Chiefs are expected to open it up a bit, but still rely mostly on running back Larry Johnson, whose 157.5-yard average leads the league in yards from scrimmage.

If the 36-year-old Green remains out for long, many critics will begin to urge the Chiefs to follow the example of the 49ers of 2005 and thrust rookie Brodie Croyle onto the field for tough but valuable game experience.

Smith had a rough time last year while throwing 11 interceptions in 165 pass attempts. But so far during a 1-2 start, the No. 1 draft pick has zero picks in 108 attempts. His passer rating of 89.8 is more than double his shaky rookie mark and he’s averaging a highly respectable 7.54 yards per attempt.

“He’s impressive,” said Chiefs defensive end Eric Hicks. “I know he had some pretty good growing pains last year he had to battle through. But this guy’s making plays this year. He has a cannon for an arm. People don’t give him enough credit as a runner, too. We’re going to have to be disciplined. He can beat you if we give him an opportunity.”

Arrowhead Stadium, one of the loudest in the league, will be a good test for Smith. Since 1994, first- or second-year quarterbacks making their initial appearance in the Chiefs’ rowdy abode are 2-17.

“It’s concerning when you’re going into a stadium where the fans are so noisy and passionate,” Smith said. “Then you add our slow starts in the last couple of games to that, and it’s a big concern for us to come out and execute from the very first series, because we can’t wait too long to get going. We’d like to start a little faster, and we’re working on that.”

The 49ers have injury problems of their own. Rookie tight end Vernon Davis is out and running back Frank Gore is slowed.

Gore was expected to be ready, however. His 266 yards rushing and three touchdowns are among the best in the league so far.

“They’ve been a big play offense averaging about 23 points a game and really in the second half is when they’re doing a lot of their damage,” said Edwards. “They’ve actually scored (51) points in the second half of football games. They’ve fallen behind early, but they’ve had a late surge and they’re third in the league in big plays. They’ve got 11 plays of 20 yards or more in passing and four of running. So they’ve had some explosion.”

With all their coming and going and changes in philosophy, the Chiefs, on the other hand, are hard to figure right now.

“With a new coach and a new quarterback on a team that’s only played two games, it’s kind of hard to sit down and say exactly what they’re going to try to do against us,” said San Francisco defensive end Bryant Young. “There are a lot of variables there. They might have some new wrinkles, so we have to be aware of that.”