The Redskins announced Sunday night that rookie quarterback Robert Griffin III did not suffer a major injury to his right knee during Sunday’s win over the Baltimore Ravens.

But the Redskins did not reveal the specifics of Griffin’s injury, calling it only a knee sprain. That description leaves open a wide variety of possibilities, and it remained unclear early Monday how long Griffin will be sidelined, if at all. The Redskins’ next game is this Sunday at Cleveland.

A knee sprain is defined as “stretching or tearing of the ligaments that support the knee,” according to the Web site for the NYU Langone Medical Center.

According to the Web site, a Grade 1 sprain involves stretching and micro-tearing of ligament tissue. A Grade 2 sprain involves partial tearing of the ligament and mild instability of the joint when it is tested. A Grade 3 sprain involves severe or complete tearing of the ligament and significant joint instability. “The more ligaments that are involved means the more severe the injury,” the Web site notes

The Redskins have not specified which ligament or ligaments are involved in Griffin’s injury, or what the severity of the sprain is.

Team spokesman Tony Wyllie said Sunday evening that Griffin, who was hurt when he was hit by Ravens defensive lineman Haloti Ngata at the end of a scramble, had undergone an MRI exam and “everything is clear.” Griffin does not have a torn anterior cruciate ligament or medial collateral ligament, Wyllie said. He called the injury a knee sprain, adding that Redskins Coach Mike Shanahan would provide further details Monday.

Shanahan is scheduled to address reporters at 3 p.m. Monday afternoon at Redskins Park.

About the same time the Redskins announced that Griffin had avoided a major knee injury, Griffin wrote on Twitter: “Your positive vibes and prayers worked people!!!! To God be the Glory!” And Griffin’s father, Robert Griffin Jr., later texted Post reporter Dave Sheinin that his son’s “knee is better than yours and mine.”

But that does not necessarily mean that Griffin will be ready to play this weekend.

According to the National Athletic Trainers’ Association, an athlete with a first-degree sprain of a joint “may be able to continue to play or will usually return to play in a few days.” An athlete with a second-degree sprain could need a brace and usually will miss 1-4 weeks, according to the NATA literature, and an athlete with a third-degree sprain of a joint could need a brace or faces possible surgery and could be sidelined for three weeks to 12 months depending on the course of treatment.

The Redskins have won four games in a row and are one game behind the first-place New York Giants in the NFC East and one game in back of the Chicago Bears and Seattle Seahawks for the NFC’s two wild-card playoff spots. The Redskins face last-place teams on the road the next two weeks. They play at Philadelphia a week after facing the Browns, before closing the season Dec. 30 at home against the Dallas Cowboys.

So the stakes are high for the Redskins, with a possible spot in the playoffs on the line. They undoubtedly want Griffin, whose play as a rookie has been so brilliant that he has been mentioned as an NFL most valuable player candidate, in their lineup for such important games. But with Griffin’s future so bright, it’s unlikely that the Redskins will take any risks with the long-term health of Griffin’s knee.