On the bright side, they're getting good at winning without him.
Stamkos scored early but then exited midway through the first period. He limped off the ice favoring his right leg following a collision with Detroit's Gustav Nyquist along the boards.
"He'll be evaluated again tomorrow," Lightning coach Jon Cooper said. "It's unfortunate that he couldn't come back, because he was helping us before he got hurt, but it is too early to speculate on anything right now."
Stamkos' goal was his ninth of the season. The six-time All-Star has 20 points in 17 games.
Stamkos missed all but one game during last season's playoffs, after having surgery for a blood clot on April 4. Despite his absence, Tampa Bay advanced to the conference finals, where it lost in seven games to Pittsburgh Penguins, the eventual Stanley Cup champions. Stamkos returned for Game 7 against Pittsburgh, a 2-1 loss.