Kansas City Chiefs linebacker Justin Houston could miss the entire 2016 NFL season after having ACL surgery on his left knee.

Last November, Houston injured his knee in a game against the Buffalo Bills. He was diagnosed with a hyperextended left knee. He missed the rest of the regular season but returned for the playoffs wearing a brace.

In February, orthopedic surgeon Dr. James Andrews examined Houston’s knee and determined that his ACL wasn’t working properly and surgery was required to fix it, according to the Kansas City Star. Houston’s recovery period is six to 12 months, meaning he could miss part or all of the upcoming season.

“Right now, he’s in the middle of the process of doing rehabilitation for an ACL injury,” Chiefs trainer Rick Burkholder said.  “It was a little tricky because his ACL wasn’t torn, but it wasn’t functioning the way it should. It’s too early to tell where he’s at right now, but he’s on the road to recovery.”

“We’re hoping, just because of his work ethic and he’s been a fast healer in the past, we’re hoping that’s not the case, that he’ll be able to play this season,” Chiefs coach Andy Reid said.

Houston signed a six-year $101 million contract extension with the Chiefs last offseason. He led the league with 22 sacks in 2014 and earned All-Pro honors. Last year, he had seven and a half sacks in 11 games, according to the Associated Press.

The Chiefs will have Dee Ford replace Houston if he were to miss part or all of the season. Tamba Hali, who recently re-signed with the Chiefs, could also fill in for Houston.