Edelman suffered the noncontact injury in the first quarter of Friday's preseason game against the Detroit Lions.
The Patriots did not disclose a specific diagnosis, but a league source confirmed to ESPN's Adam Schefter that Edelman suffered a torn ACL. The full extent of the injury was first reported Saturday by The MMQB.
"Julian is a great competitor, works hard, is tough, does all the things that you ask him to do. So I feel badly for him, of course," Patriots coach Bill Belichick said Saturday. " ... We hope that he will have a speedy and complete recovery, but the team will have to move on and compete this season without him. It's unfortunate, but that's the way it is."
From the standpoint of offensive production, Belichick said the team "has to move forward with the resources we have. Unfortunately, we've lost one. An important one. But as a team we still have to move forward and that's what we'll do. That process has already started."
Patriots quarterback Tom Brady posted a photo on Instagram on Saturday of him hugging Edelman with a one-word caption: "Gladiator."
Edelman, 31, suffered the injury while running an in-breaking route, beating linebacker Jarrad Davis and catching the ball at Detroit's 29-yard line. Edelman then turned up the field and planted his right foot into the turf at the 17 to avoid safety Tavon Wilson and lunged forward. He immediately reached to the back of his right knee.
Edelman walked off the field limping but unassisted. After a few minutes of being looked at in a makeshift tent, he was taken by cart to the locker room.
Edelman, who in June signed a two-year contract extension through 2019, has 436 receptions (including playoffs) since 2013 -- 187 more than any other Patriots player in that span (Rob Gronkowski, 249).
Brady is losing one of his most reliable pass-catchers. Edelman has caught 397 career passes from Brady, third most in team history, according to Elias Sports Bureau.
Since 2009, Edelman's first season with the Patriots, New England is 84-19 (.816) in games Edelman has played and 15-10 (.600) without him, according to ESPN Stats & Information.
ESPN's Mike Reiss and Michael Rothstein contributed to this report.