The Tampa Bay Lightning will be without their best defenseman when they try to stave off elimination.
Hedman also missed Game 3 of the first-round series Sunday with an undisclosed injury.
The reigning Norris Trophy winner notched 54 points in 70 regular-season games during 2018-19.
Cooper added Tuesday that Anton Stralman will also miss Game 4. Stralman has been out with what the club classified as a lower-body injury and last played in Tampa Bay's penultimate regular-season game on April 4.