Lehner recently returned from a knee injury. ESPN is speculating that the season-ending surgery is on his knee.
Lehner played five games after returning on April 3 from what the team termed a "lower-body injury."
Although Lehner struggled with an .889 save percentage in five appearances since his return, backup Logan Thompson has won seven of his past 10 decisions, with a save percentage of .922.
Lehner's play was criticized by Knights coach Pete DeBoer, and he was pulled after the first period of Wednesday's overtime win over the Washington Capitals after stopping 12 of 13 shots.