Source: MRI reveals Vikings' Jackson has sprained MCL

An MRI revealed that Vikings quarterback Tarvaris Jackson suffered a sprained medial collateral knee ligament during Minnesota's 23-15 victory over the Ravens on Saturday night, according to a source.

The MCL injury could keep Jackson out of the Vikings' preseason game this coming Saturday against the Steelers, but the injury isn't considered serious enough to sideline him for the start of the regular season.

Vikings coach Brad Childress labeled the injury a contusion. Jackson was hit in the knee while making a run toward the right sidelines in the first quarter.

Jackson, who was 7-of-11 passing for 82 yards, stayed in the game a couple of plays before the pain in the knee forced him to the ground.

Childress might keep Jackson out of the Vikings' final two preseason games to ensure that he is healthy for the regular-season opener against the Packers.

Senior writer John Clayton covers the NFL for ESPN.com.