Hertl, perhaps the team's best forward through the first two games of the finals, is recovering from a lower-body injury, coach Pete DeBoer announced Saturday morning at the SAP Center.
"He's out tonight," DeBoer said as the Sharks prepared for the first Stanley Cup finals game in San Jose, adding "He's arguably been maybe our best player through the first two games."
The Sharks trail the Penguins 2-0 in the best-of-seven series, having lost two one-goal games in Pittsburgh.
Hertl normally plays on the Sharks' top line with captain Joe Pavelski and Joe Thornton. The 22-year-old native of Prague has six goals and five assists in the playoffs and was a strong presence in the two games in Pittsburgh, including scoring a goal in Game 1. He took a hard hit from Pittsburgh forward Patric Hornqvist in the third period of Game 2.
"Injuries are no excuse this time of year," DeBoer said. "We've got someone that will go in for him, and we'll roll out there and be ready to play."
Hertl skated in a tracksuit before the Sharks' practice Friday but did not join his teammates.
Melker Karlsson skated in Hertl's place on the wing with Pavelski and Thornton at Friday's practice and told ESPN he welcomed the challenge of playing with those top players if called to do so.
"I played with them before last season quite a bit," Karlsson said. "I know how they play, and I know what I'm going to bring there if that's the case.
"They're good players," the 25-year-old added. "Obviously they're one of the best in the league, and they spend much of their time in the offensive zone usually. You've got a lot of opportunities to score too when you play with those two, so that's a fun part."