Penguins coach Mike Sullivan said before the game that Malkin, who has a lower-body injury, did not experience a setback. He said it's "just part of the process."
Sullivan said Malkin could return for Game 3 of the series.
Malkin missed Game 6 of the Penguins' opening-round win against the Philadelphia Flyers and Game 1 against the Capitals with the injury.
He led the Penguins in goals and points during the regular season. He had three goals and two assists against the Flyers before suffering the injury when he got tangled with the Flyers' Jori Lehtera in Game 5.
Winger Carl Hagelin also remained out for the Penguins with an upper-body injury.
The best-of-seven series is tied 1-1.