NFL Network's Ian Rapoport first reported the news.
This is the second major injury to a Patriots wide receiver after Julian Edelman suffered a season-ending torn ACL.
Although the depth chart hasn't always been conducive to big numbers, Mitchell had a promising rookie season with 32 catches for 401 yards and four touchdowns. In a year where key receiving targets like Rob Gronkowski, Danny Amendola and others dealt with injuries, Mitchell came up big down the stretch to help the Patriots keep winning.
He then came through with six catches for 70 yards in the Super Bowl LI win over the Atlanta Falcons.
While playing time hasn't always been there for the former fourth-round pick, he continues to show plenty of promise when given the opportunity.
Unfortunately, knee injuries have been a problem for him since before his rookie year, and now it appears a new issue could keep him off the field.
The good news for New England is the squad still has options at receiver, even without Mitchell and Edelman. Brandin Cooks and Chris Hogan should lead the way while giving Tom Brady consistent options in the passing attack, while New England also added 2015 first-round pick Phillip Dorsett.