Halak missed the first four games due to an undisclosed upper body injury, with the Islanders going 2-1-1 in his absence. Backup Thomas Greiss was 2-0-1 with a 2.64 goals-against average and a .925 save percentage.
The Western Conference-leading Sharks came at 4-0-0, one of two unbeaten teams in the league.
Halak, 38-17-4 with six shutouts and a 2.43 goals-against average last season in his first year with the Islanders, left New York's preseason opener on Sept. 21 after two periods and had not played in a game since.
Defenseman Thomas Hickey also made his season debut. He was injured in a preseason game on Sept. 28 against Washington and was expected to miss up to four weeks.
Forward Eric Boulton was placed on injured reserve with a lower-body injury.