Vegas Golden Knights defenseman Alex Pietrangelo has taken a costly roughing penalty in back-to-back playoff games, and Bruce Cassidy has had enough.

"Veteran guy should know better," the Golden Knights coach told reporters following Vegas' 3-2 Game 5 loss to the Dallas Stars.

Pietrangelo didn't speak to the media.

After Tyler Seguin received a minor penalty for an illegal check to the head of Golden Knights rearguard Shea Theodore during the second period, Pietrangelo was penalized later in the frame for swatting his hand into the Stars forward's face.

The contest was tied 2-2 at the time, but Jason Robertson scored the eventual game-winner on the ensuing power play.

Pietrangelo was similarly in the box during a crucial moment in Game 4. After the two-time Stanley Cup champion was penalized for roughing Roope Hintz away from the play, the Stars scored on the man advantage for the first of three unanswered goals during their 4-2 victory.

When asked if he found Pietrangelo's recent lack of discipline surprising or disappointing, Cassidy said "both."

"There's controlled aggression out there all over the ice in every series. ... There's a little bit of a learning curve that goes into it," the bench boss explained. "We should know better, but it happens. ... Margins are too thin."

"We have to take care of our discipline," he added. "We just have to. It's that simple."

Pietrangelo earned zero penalty minutes through the first three games of the series. He was assessed just one minor penalty when these two teams met in the conference finals last spring.

The 34-year-old racked up 24 penalty minutes in 64 games during the regular season.

Pietrangelo has been one of the Golden Knights' key penalty killers since he arrived in Sin City back in 2020. He averaged the second-most shorthanded ice time (2:27) on the team during last year's postseason, trailing only Brayden McNabb.

Vegas won the first two games of its first-round series against the Stars, but the Western Conference's top seed stormed back with three straight victories to put the reigning champs on the brink of elimination.

"We'll hang our hats on the fact that we're the defending Stanley Cup champions," Cassidy said. "There's lots of resolve in that room, a lot of winners in that room, a lot of pride in that room."

Game 6 is scheduled for Friday at T-Mobile Arena.

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