An MRI performed Monday revealed the full extent of the running back's injury. The Boston Globe earlier reported Lewis' diagnosis.
The Patriots announced Lewis has been placed on season-ending injured reserve but did not publicly disclose the nature of the injury.
Lewis vowed to return "like I never left" in a tweet Monday night:
— Dion Lewis (@DionLewisRB) November 10, 2015
Patriots coach Bill Belichick said Tuesday that the team would miss the versatile back.
"It's a tough break for Dion," he said. "Dion has worked really hard, had a really productive spring, [training] camp, and helped our football team a lot. Great attitude, great worker, great kid. Hopefully he'll have a good recovery and we'll get him back [next year]."
Lewis was hurt with 5:40 remaining in the third quarter of Sunday's 27-10 win over the Washington Redskins after being tackled from behind by Redskins defensive lineman Chris Baker at the end of a 14-yard catch-and-run. Television replays showed him reaching for the area of his left knee as the play was ending.
Lewis, who has been a spark plug for the Patriots with his sharp cuts and knack for stopping and starting in an instant, had remained down on the ground as the medical staff came onto the field. He eventually walked off the field slowly under his own power and headed straight to the locker room for medical tests.
"It's an unfortunate situation for him. He's given our team a lot of great performances each and every week," said second-year running back James White, who is the top candidate to replace Lewis. "We pray for him and the rest of us backs have to step up."
Selected by the Philadelphia Eagles in the fifth round of the 2011 draft out of Pitt, Lewis joined the Patriots this past offseason. He signed a two-year extension with the Pats in October that with incentives could ultimately be worth approximately $5 million.
Lewis finishes the 2015 season with 234 yards rushing and two touchdowns to go along with 388 receiving yards and two receiving touchdowns for the 8-0 Patriots.
ESPN.com Patriots reporter Mike Reiss contributed to this report.