The 31-year-old star is scheduled for an MRI Monday on his left knee, Patriots Coach Bill Belichick said.
The knee has serious ligament damage, Yahoo! Sports said, citing a source on the team.
"It's bad," a source said in the Yahoo! report. "We're going to have to play without him."
Brady was injured in the first quarter of the Patriots' 17-10 win over Kansas City. He was helped from the field, left the game and did not return.
Matt Cassel took over as quarterback for the Patriots and passed for 152 yards and a touchdown.
The Patriots have been to four Super Bowls, winning three of them, with Brady.