Cooper added he's unsure if it will be a long-term issue, according to John Dietz of the Daily Herald.
Stamkos played just 5:17 but remained on the bench for the entire game. He wanted to be there to support his teammates during the 4-2 victory, Cooper said, according to Bryan Burns of NHL.com.
The Lightning are already without reigning MVP Nikita Kucherov as he deals with an upper-body injury he suffered Tuesday.
Stamkos has posted seven goals and 13 assists in 19 games.