Disappointing offensive production from several key players prompted the Royals to fire hitting coach Kevin Seitzer on Thursday morning — less than 12 hours after the club completed a 72-90 season.

Seitzer, 50, just finished his fourth season as the club’s hitting coach and was the longest-serving member of the club’s on-field staff.

“I just felt the offense underperformed all year long,” manager Ned Yost said. “Kevin Seitzer has tremendous passion, tremendous energy and worked his tail off. But we just felt like there’s more offense in there. We just felt it was time to make a change.”

The contracts of pitching coach Dave Eiland, bench coach Chino Cadahia, third-base coach Eddie Rodriguez and first-base coach Rusty Kuntz were extended through 2013.

The Royals still have a vacancy at bullpen coach, but are expected to fill it by promoting either Doug Henry from Class AAA Omaha or Larry Carter from Class AA Northwest Arkansas. Both filled that role on an interim basis in the season’s closing weeks.

“We’ve got two real good in-house candidates,” Yost said. “We’ll talk about that, too. We’re not ready to make a decision. The season just ended, and we need time to make sure we completely think through it.”

The bullpen position became vacant when Steve Foster, who held the role since 2010, shifted on Aug. 31 to become the organization’s minor-league pitching coordinator.

Seitzer paid a price for disappointing seasons by two of the club’s young cornerstone players, first baseman Eric Hosmer and third baseman Mike Moustakas, and a sharp decline by veteran right fielder Jeff Francoeur.

“Kevin was a tremendous hitter when he was in the big leagues,” Yost said. “His philosophy was, basically, to stay to the middle of the field and to the off side. I think we’ve got a group of young power hitters who are capable of hitting homers.”

The Royals finished near the bottom of the American League in many key offensive categories: 12th among the 14 teams in runs scored, 13th in home runs and 14th in walks.

“Our offense was built more around singles and doubles,” Yost said, “but it’s difficult to get three or four singles in a row to score a run. We have to have the ability to open it up a little more, use the power that we have to take advantage of a quick strike.

“A walk, a base-hit and boom — there’s three runs. I think that’s the major difference in philosophy.”

Yost said he’d “like to stay in-house” in finding a replacement for Seitzer.

That suggests the candidate pool will include Omaha hitting coach Tommy Gregg, Northwest Arkansas hitting coach Terry Bradshaw, Rookie Surprise hitting coach Andre David and minor-league hitting coordinator Jack Maloof.

David served as the Royals’ hitting coach from May 30, 2005, through May 1, 2006. Maloof spent three years as the Marlins’ hitting coach. Gregg and Bradshaw have never coached in the big leagues.