The Chargers confirmed the diagnosis Monday after Ian Rapoport of NFL Network reported Gordon should return before the end of the regular season, provided he does not suffer any setbacks in rehab earlier in the day.
Gordon, 25, went down in Sunday's win over the Arizona Cardinals after rushing for 60 yards and two touchdowns. He suffered the injury when Cardinals defensive tackle Robert Nkemdiche hit him in the leg and took him to the ground on a reverse play in the third quarter.
The Chargers lost 10 yards and their top running back on the play.
"That's a play that we've had in for a long time," Chargers coach Anthony Lynn told reporters. "We've been trying to get it called in a game. Melvin loves the play, and we were not taking our foot off the gas pedal.
"It's a play if he gets around the perimeter, he may still be running. But they made a heck of a play and hit us in the backfield."
The Chargers will likely hand starting reins over to Austin Ekeler with Gordon out. Ekeler has been one of the league's more effective backups, totaling 763 yards and four touchdowns in 11 games. He has been particularly good in pass-catching downs, putting up 32 grabs for 354 yards and three scores.
"I prepare every week like I'm going to be starting," Ekeler said. "If I'm getting more reps, then that's what it is. I have to pick up the slack for Melvin.
"Those are big shoes to fill. Melvin and I are different running backs. He's a beast, so I have to step up my game."
Justin Jackson and Detrez Newsome could also get some work as backups. Jackson rushed for 57 yards on seven carries against the Cardinals.