Backstrom, 30, seemed to suffer a hand injury in Game 5 when blocking a Justin Schultz shot. Coach Barry Trotz had previously told reporters he expected Backstrom to play.
Third-line center Lars Eller will be moved up to the second line as a result of Backstrom's absence. Travis Boyd will move up to the third line.
The Capitals hold a 3-2 lead in the best-of-seven series heading into Game 6. They're looking to unseat the two-time defending champions and overcome their own postseason demons. Washington has been eliminated by Pittsburgh each of the last two seasons.
Backstrom scored five points through the first five games, including a three-assist effort in Game 3. He finished with 71 points during the regular season.