The Kansas City Chiefs were the AFC's most improved team last season, and don't expect to be taking a step back anytime soon.

For the Buffalo Bills, there's hope, but likely another year of rebuilding.

The Chiefs look to get off to a strong start Sunday when they host the Bills, who believe they have turned the corner after three straight last-place finishes.

Kansas City went from a 4-12 record in 2009 to 10-6 last season, the franchise's best finish since 2005. The Chiefs went into the playoffs as a division winner for the first time since 2003, but were eliminated by Baltimore in a wild-card matchup.

"This will be a great opener for us," coach Todd Haley said. "I'm excited to be at Arrowhead in front of our fans and ... I know the players are really excited."

Quarterback Matt Cassel made great strides in his second year with club. He threw for 3,116 yards with 27 touchdowns and only seven interceptions in 15 games. Cassel's plus-20 touchdown-to-interception differential trailed only New England's Tom Brady, who was plus-32.

Cassel injured his ribs after being sacked during a loss to Green Bay in the preseason finale, but will be under center Sunday.

"He threw me some passes in practice, he's moving around great, throwing the ball good," fullback Le'Ron McClain said. "I asked him, was he OK when we got back here, and he said he's good, and that's all I got to hear. He'll be ready."

Wideout Dwayne Bowe was Cassel's top target last season with a NFL-best 15 touchdown receptions and career-high 1,162 yards. However, tight end Tony Moeaki, who finished second with 556 receiving yards as a rookie, is out for the year with a knee injury.

"He had a tremendous year last year ... and was a guy that had a heavy presence for us over the middle," Cassel said.

With Moeaki shelved, Cassel may be forced to rely more on his backfield. The duo of Jamaal Charles and Thomas Jones showed they were more than up to the task in 2010.

Charles finished second in the league with 1,467 rushing yards, and first among running backs with an average of 6.4 yards per carry. He ran for a season-high 177 yards in the Chiefs' 13-10 overtime win against Buffalo last Halloween.

The well-traveled Jones added 896 yards and six touchdowns in 2010, his first season with Kansas City after stops in Arizona, Tampa Bay, Chicago and with the New York Jets.

The 2,363 yards by Charles and Jones where the most by any NFL duo.

Kansas City's defense is led by linebackers Derrick Johnson, who had a team-high 96 solo tackles, and Tamba Hali, whose 14 1/2 sacks were good for second in the league behind Dallas' DeMarcus Ware.

Cornerback Eric Berry, the team's first-round pick a year ago, was fourth among rookies with four interceptions.

Meanwhile, there is cautious optimism on a Buffalo team that has seen its win total drop in each of the last three years and hasn't been to the postseason since 1999. Buffalo is 1-4 in its last five season openers, and has the fourth-lowest winning percentage (.392) among current franchises in opening games.

Ryan Fitzpatrick begins this season as the Bills' unquestioned starting quarterback. A seventh-round pick by St. Louis in 2005 from Harvard, Fitzpatrick passed for 3,000 yards with 23 touchdowns and 15 picks in 13 games last year after supplanting Trent Edwards.

"I feel like I'm always going to have to go out and prove myself," he said. "I'm never going to get the benefit of the doubt. I'm not a first-round pick, I'm not a big-school, big-name guy."

Fitzpatrick will be looking for Steve Johnson, who had 1,073 receiving yards and 10 touchdowns last year. Longtime veteran Lee Evans was dealt to Baltimore, leaving former Jet Brad Smith, Donald Jones, Roscoe Parrish and David Nelson to pick up the slack at wide receiver.

Fred Jackson ran for 927 yards a year ago with five of the team's six rushing scores. Ex-Clemson star C.J. Spiller, who rushed for 283 yards as a rookie, will need to show improvement.

Still, coach Chan Gailey likes what saw in the preseason.

"We are much more advanced as an offense, we know more of what we're doing," he said.

The Bills have to hope things will improve on defense, which was last in the league against the run (169.6 yards per game).

Marcell Dareus, a massive defensive end and the club's first-round pick from Alabama, joins nose tackle Kyle Williams and Dwan Edwards on the front line.

Shawne Merriman, meanwhile, is healthy again and hopes to recapture the success he had early in his career.

The 27-year-old linebacker has had just four sacks in three injury-plagued seasons from 2008-10. That came after three Pro Bowl seasons in which he totaled 39 1/2 sacks for San Diego.

Buffalo has won three of the last four meetings with Kansas City.