Cain, considered a top prospect in the organization, suffered the injury in the eighth inning of the Huntsville Stars' game at Carolina while attempting to make a diving catch.
"I was in a 'no doubles' situation, playing deep, and on a fly ball I ran in and made a sliding catch on my knees," Cain said on Sunday while at the airport awaiting a flight to Milwaukee, where he was scheduled to undergo an MRI on Monday. "The outfield there is kind of hard, and I banged my knee against the ground."
Cain actually stayed in the game after the injury and came to the plate for his next at-bat, but when he hit a line drive to center, he felt his knee go out on him as he broke out of the box. Although he is able to walk without aid, he said it hurts to move the knee.
According to reports, the prognosis of the doctor on the scene at the game was a possible torn medial collateral ligament which, if confirmed, could sideline Cain for the rest of the season.
Cain had just been activated by Huntsville earlier in the week after missing the beginning of the season with a hamstring injury. He had gone 0-for-14 in his first four games for the Stars.
Rated the top outfield prospect in the organization, he combined to hit .279 with 11 homers, 58 RBIs and 25 steals along with 31 doubles at three levels in 2008. He finished off 2008 strong, hitting .333 with five home runs and 11 RBIs in 18 games in the Arizona Fall League.
Lisa Winston is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.