Ravens coach John Harbaugh called the injury "a crazy circumstance on AstroTurf." Fuller, a two-time Pro Bowl cornerback, was hurt covering Elijah Moore with 1:18 left in Baltimore's 24-9 win over the New York Jets at MetLife Stadium on Sunday.
"He got pushed off and just landed the wrong way," Harbaugh said. "And the turf doesn't usually give as much as grass. So that's what happened."
Fuller, 30, was playing in his first game for his hometown team. Ravens officials initially thought Sunday there was a chance that Fuller's injury wasn't serious, but tests Monday confirmed the ligament tear. Fuller signed a one-year, $2.5 million deal with Baltimore this offseason.
Before the Sunday game, Peters was spotted running the stairs of MetLife Stadium.
Asked how close Peters was to playing, Harbaugh said, "I don't have the measurement on that."
In recent years, the Ravens have had trouble keeping their cornerbacks healthy. Last year, Baltimore had four cornerbacks finish the season on injured reserve.
In addition to Fuller, the Ravens also lost left tackle Ja'Wuan James for the season in the opener Sunday. He tore his left Achilles tendon late in the second quarter.