Steve Young wants to see Chiefs quarterback Alex Smith crank it up

The Kansas City Chiefs (9-0) are the NFL’s only undefeated team, and quarterback Alex Smith is playing it close to the protective vest. He’s ranked 27th in passing yards, yet has a league-low four interceptions.

Kansas City’s defense leads in several categories — sacks, takeaways, and efficiency in the red zone and on third downs –- but its keep-it-under-the-speed-limit offense hasn’t scored a touchdown in the last six quarters.

Sunday night’s showdown at Denver (8-1) pits the Broncos’ top-ranked offense, averaging 458.7 yards per game, and a Kansas City defense that has surrendered a league-low 12.3 points per game. The Chiefs are only the second team since 1970 that hasn’t given up more than 17 points through the first nine weeks.

Despite acknowledging that you don’t mess with what’s working, at least one ardent — and informed — Smith supporter would like to see Coach Andy Reid pry open his playbook a little wider for his quarterback.

“What Andy’s asked Alex to do, Alex has done really well,” said Hall of Fame quarterback Steve Young, another former San Francisco 49ers quarterback. “The real question I have, what I’d really love to see — and maybe what Alex would love to see — is, if you started calling more expansive plays and having a lot more fun, would [Smith] answer the call?

“In San Francisco, he started to spread his wings. I don’t know if Alex or we have seen the best of Alex Smith, and it’s been pretty dang good. … It would be fun to see if they just let it rip and let him throw it 45 times a game.”

Young said the suggestion that Smith is merely a “game manager” whose job requires that he simply not turn over the ball is dismissive of what he has done so far.

“He understands what the team and coach are trying to accomplish,” said Young, an ESPN analyst. “He’s a really smart guy, and he doesn’t have a huge ego. So he just does a great job of making sure that he figures out the calculus to make sure he wins. People don’t understand that. That’s its own art.”

This weekend marks a significant turning point in Smith’s career. It was in Week 11 last season that Colin Kaepernick made his first start for the 49ers, stepping in while Smith recovered from a concussion. It was a job that Kaepernick, the quintessential pass-run threat, wouldn’t relinquish, even though Smith was having a spectacular season when he was injured.

Kaepernick wound up taking the 49ers to the Super Bowl; Smith was traded to Kansas City.

This season, Kaepernick and Smith have remarkably similar numbers. Both have thrown nine touchdown passes, tied for 27th in the NFL, and Kaepernick’s six interceptions put him in a seven-way tie for second place behind Smith. The 49ers are ranked last in the league in passing yards, averaging 173.9 per game.

San Francisco’s injury-riddled receiving corps has left Kaepernick with the same challenge Smith faced early in his career with the 49ers — not enough sure-handed targets. What’s more, there are whispers that a foot injury has curtailed the kind of explosive runs we saw from Kaepernick last season.

“It’s not nearly as expansive as it was late last season,” Young said of the 49ers’ offense. “And so you wonder if Colin’s just responding to having less asked of him, and so you get less of him, or if there’s something else. I think we’re going to get another piece to that puzzle this weekend [at New Orleans], because certainly they can’t just run the ball and play good defense this weekend.”

This figures to be a telling Sunday for the Chiefs and 49ers and their quarterbacks, whose careers will be linked forever.

Third choices

Oakland announced quarterback Matt McGloin, an undrafted rookie, would start at Houston in place of the injured Terrelle Pryor, meaning the list of teams that have started three quarterbacks this season continues to grow. Buffalo, Cleveland and Minnesota have started three each, and Green Bay is on its third.

Cutting it close

So far, 100 games have been within seven points in the fourth quarter, the most through Week 10 in league history.

In another indication of competitive balance, in seven of the eight divisions, at least one team is in first place (or tied for first) that didn’t win its division last season.

Nice timing

Seattle Seahawks receiver Percy Harvin is listed as probable for Sunday’s game, just in time to face his former Minnesota Vikings.

“I wish it was one more week he was sitting out,” Vikings running back Adrian Peterson said, jokingly. “But no, I’m excited to see him. I’m excited that he can get back to doing what he loves to do.”

Harvin hasn’t played since Nov. 4, 2012 — against the Seahawks. An ankle injury cut short his 2012 season, and, after being traded to Seattle, a hip injury required surgery in August that kept him out to this point.

sam.farmer@latimes.com

Twitter: @LATimesfarmer