Coach Jack Capuano announced Halak will be out "at least six weeks" after suffering a lower-body injury late in Tuesday night's win over the Pittsburgh Penguins.
"We'll go from there," Capuano said.
Halak is in danger of missing the first round of the postseason and possibly more.
The 30-year-old Czech goalie has an 18-13-4 record, a .919 save percentage and a 2.30 goals-against average.
Halak will be replaced in the lineup by Thomas Greiss, who has been a pleasant surprise in his first season with the Isles. Greiss has an 18-6-3 record, .930 save percentage and 2.20 goals-against average.
The Islanders play at Toronto on Wednesday night.
Jean-Francois Berube will serve as Greiss' backup.
The injury to Halak comes just as the Islanders are enjoying one of their best stretches of the season. They have won seven of their last eight games and have been closing in on the rival Rangers for second place in the Metropolitan Division. (The Rangers hold a three-point advantage.)
The timing is also unfortunate for the Islanders because the trade deadline passed last week.
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