Kansas City, Mo. ? The season is over for Jamaal Charles.

The All-Pro running back will go on injured reserve with a left knee injury, leaving the winless Kansas City Chiefs without their best offensive player.

Charles, the NFL’s second leading rusher last season, suffered the injury after taking an awkward step finishing off a run in the first quarter of Sunday’s 48-3 loss to the Detroit Lions.

Charles had an MRI exam Monday, and Chiefs coach Todd Haley said he would join tight end Tony Moeaki and Pro Bowl safety Eric Berry on injured reserve. Moeaki tore his left ACL in Kansas City’s preseason finale against Green Bay, while Berry tore his left ACL in a 41-7 season-opening loss to Buffalo.

“Injuries are part of the game,” Haley said. “Sometimes through preparation, sometimes just through freak happenings, and we’ve seen a little of each.”

The Chiefs are off to one of the worst starts in NFL history, getting outscored 89-10 by a pair of teams that combined to win 10 games last season. The miserable start has caused talk radio shows to light up with calls for Haley and general manager Scott Pioli to be fired, even though the franchise is coming off a surprising 10-6 season in which it won the AFC West title.

The Kansas City offense has turned the ball over nine times and is averaging 240 yards per game, 30th of 32 teams in the NFL. The defense has been just as bad, allowing nearly 390 yards and a point total exactly four touchdowns more than the league’s second-worst defense.

Haley vowed that the Chiefs would keep working to turn things around, but that won’t be easy after losing arguably three of their five best players to season-ending injuries.

“You hate to see any of your teammates, whether it’s Jamaal, Eric Berry, Tony Moeaki, any one of those guys go down, because we’re not just teammates, we’re friends,” running back Thomas Jones said. “You hate to see a guy that works as hard as Jamaal go down like that.”

Charles struggled along with the rest of the Chiefs in their opener against Buffalo, gaining 56 yards on 10 carries. But he appeared to be finding his stride on Kansas City’s opening drive against the Lions, taking his first carry around left end for 24 yards.

He got another carry three plays later, this time headed toward the right, and took an awkward step out of bounds after a three-yard gain before colliding with the Lions’ mascot.

Charles immediately grabbed at his left knee and rolled around on the ground before the training staff finally reached him. He was loaded onto a cart and taken to the locker room, and the team initially said his return was doubtful — even though it was clear that he was seriously injured.

How serious wasn’t revealed until Monday.