Vikings' quarterback Tarvaris Jackson suffered a sprained medial collateral knee ligament on Saturday night, according to ESPN.com's John Clayton. The injury could keep Jackson out of the team's preseason game against the Steelers next weekend, but the injury isn't considered serious enough to sideline him for a long period of time. It's possible Coach Brad Childress could keep Jackson out of the Vikings' final two preseason games to ensure he's healthy for their regular-season opener against the Packers.