Gronkowski was injured in the third quarter of Sunday's 27-26 New England win over Cleveland and was taken from the field on a cart. Subsequent examinations indicated torn anterior cruciate and medial collateral ligaments in his right knee, NFL.com said.
Gronkowski played in just seven of the Patriots' 13 games but was fourth on the team with 39 receptions and second with 592 receiving yards. He is one of three New England players with four touchdown catches, trailing only Julian Edelman, who has five, for the team lead.
The 24-year-old tight end missed the first six games of the season due to back and forearm surgeries during the off-season.
Over his four-season, 50-game NFL career, Gronkowski has caught 226 passes for 3,255 yards and 42 touchdowns. He's been selected for the Pro Bowl twice.
The Patriots were down 12-0 when Gronkowski was injured but rallied to beat the Browns, scoring twice in the final 61 seconds.
New England is 10-3 this season.