Koekkoek, who plays for the Windsor Spitfires of the Ontario Hockey League, was injured Thursday in a fight during a game against the Saginaw Spirit.
Windsor coach Bob Boughner termed Koekkoek's issue an upper-body injury.
Friday, Lightning general manager Steve Yzerman told the Tampa Tribune that Koekkoek had injured his shoulder and more information would be known Monday.
If Koekkoek is unable to play it would mark the third straight season he's had ended prematurely with a shoulder injury. In the previous two, Koekkoek needed surgery on his left shoulder; the injury sustained Thursday is reported to be to his right shoulder.
In 62 games this season, Koekkoek has 15 goals, 53 points and a plus-44 rating. He's tied for third among OHL defensemen in goals and ranks third in points. He is tied for fifth in the OHL in plus/minus.