An MRI on Monday confirmed the initial diagnosis, and there is no ligament damage to the ACL, a league source told ESPN NFL Insider Adam Schefter.
Injuries such as these typically sideline players two to six weeks. There are four games left in the season.
Forte walked off the field with athletic trainers in the first quarter after taking a shot to the right knee. The club immediately declared him out. As he left Soldier Field after the game, he said it was "just a sprain. I'll be fine."
"It's not a good feeling when you see your star tailback go down with a knee injury, but it's part of the game," coach Lovie Smith said. "We'll just keep our fingers crossed, and hopefully it's not that serious."
Forte played with a sprained MCL in 2009, when he played in all 16 games but had the lowest number of yards and carries in his career.
With approximately six minutes remaining in the first quarter, Forte took a handoff off left tackle and was hit in the knee by Chiefs linebacker Derrick Johnson after a run for no gain.
Forte walked off the field under his own power, and sat down behind the team's bench area, where the team's athletic trainers examined the knee. As trainers looked at the knee, quarterback Jay Cutler came over and appeared to offer words of encouragement along with backup running back Marion Barber.
Minutes later, Forte walked to the locker room with trainers and the team announced he was out.
A fourth-year veteran, Forte came into Sunday's contest with 985 yards rushing and 490 yards receiving. Forte was leading the NFL in yards from scrimmage, and left the game Sunday having rushed for 12 yards on five attempts.
With the team fighting for a playoff spot with a backup quarterback at the helm in Caleb Hanie, Forte's injury couldn't come at a worse time.
"When we [lost] our quarterback, we're like, 'OK, we'll be fine.' Then we lose our star of the team, star of the offense. That's tough, " said Roy Williams. "[Forte's] our screen guy. He leads the team in receptions. He's a versatile back, top-three back in the league. When he goes down, it's tough to fill those shoes. This offense runs through him."
Prior to the injury, Forte was attempting to negotiate a long-term extension with the team. According to sources, Forte had turned down a deal worth between $13 million and $14 million.
But the team didn't respond with a more lucrative offer, and the sides hadn't been able to agree to a new deal, and thus mutually decided to suspend contract negotiations.
Information from ESPNChicago.com's Michael C. Wright and ESPN NFL Insider Adam Schefter was used in this report.