DeBoer added that Sharks captain Joe Pavelski will be a game-time decision.
Karlsson, Hertl, and Pavelski all left Game 5 with various ailments and did not return.
Karlsson's been dealing with a nagging groin injury that cost him 29 games during the regular season. Despite the ailment, he's tied for second on the Sharks with 16 points during this playoff run.
The extent of Hertl's and Pavelski's injuries are unclear, but they both took hard hits in Game 5. Hertl is second on the Sharks with 10 goals during the postseason. Pavelski missed the first six games of San Jose's second-round series with the Colorado Avalanche, but still has nine points in 13 playoff games.
The Sharks have lost back-to-back contests and trail the Blues 3-2 in the Western Conference Final series.