With 7:35 left in a scoreless first, Bishop, a finalist for the Vezina Trophy as the league's top goaltender, was scrambling around the Tampa Bay net during a Pittsburgh forecheck and seemed to hurt himself as he fell to the ice. Bishop immediately grabbed at his left knee.
It did not appear from replays as though any other player made contact with him.
Trainers attended to Bishop, and even though he sat up, they called for a stretcher, and he was taken from the ice. Bishop, 29, was replaced by backup Andrei Vasilevskiy, who closed out Tampa Bay's 3-1 win.
Tampa Bay did not have immediate word on Bishop's condition.
The Lightning also briefly lost center Tyler Johnson late in the first period after he suffered an apparent injury to his lower body.
Johnson was injured after he went down on a hit into the boards. He had to be helped off the ice but returned in the second period.
Letang left the ice but returned to the bench during the injury timeout when Bishop was being attended to. Callahan remained in the game but is likely to face supplemental discipline from the league's Department of Player Safety.