Coach Leslie Frazier will give an update of the injury during his 12:30 p.m. CT news conference on Monday.
A source with knowledge of the situation told ESPN's Josina Anderson that the MRI also revealed that Peterson tore the MCL in his left knee as well. According to the source, the team intends to confirm the torn MCL at Monday's news conference.
Peterson said earlier Saturday that he knew "it was something bad" the moment he was hit and that he was in "severe pain" as he lay on the field.
He was hurt when tackled by Redskins safety DeJon Gomes at the end of a 3-yard gain on the first offensive play of the second half.
"Anytime you take a blow to the knee like that, you're concerned about the ACL, MCL," Peterson said as he stood on crutches in the locker room. "I'm trying to stay as positive as I can."
A torn ACL will likely sideline Peterson for some nine months and make it difficult for him to return for the start of next season.
On the play after Peterson was hurt, Minnesota's Christian Ponder suffered a head injury when sacked by Adam Carriker and London Fletcher. Ponder remained in the game for one more play -- a third-down incomplete pass -- before heading to the locker room.
Frazier said Ponder had "concussion-like symptoms." The coach said he's uncertain whether the quarterback will play in next week's season finale against the Chicago Bears.
"We'll have to weigh it when we get back," Frazier said, "and I'll follow the doctor's lead."
Peterson had 12 carries for 38 yards when he left. He also had a 1-yard touchdown run in the second quarter.
Ponder completed 8 of 13 passes for 68 yards. He was replaced by Joe Webb, who scored on a 9-yard run on the next series to give the Vikings the lead.
Information from The Associated Press was used in this report.