Islanders goaltender Semyon Varlamov watches the puck curing the third...

Islanders goaltender Semyon Varlamov watches the puck curing the third period of the team's NHL game against the Penguins on Wednesday at UBS Arena. Credit: AP/Frank Franklin II

The Islanders’ last loss showcased the persistent flaws in their defensive game. But Thursday’s practice exposed the biggest weakness in the organization.

Goalie Semyon Varlamov was not on the ice at Northwell Health Ice Center in East Meadow because of maintenance, and coach Lane Lambert said he is day-to-day. It’s unclear whether Varlamov will be able to back up Ilya Sorokin against the Capitals on Friday night at UBS Arena after the Islanders came out of their three-day Christmas break with an abysmal 7-0 loss to the visiting Penguins on Wednesday.

If either Sorokin or Varlamov were seriously hurt and missed significant time, it could very well keep the Islanders out of the playoffs because there is not another NHL-ready goalie in the organization.

Minor-league journeyman Ken Appleby — who played three NHL games for the Devils in 2017-18 — was recalled from the Islanders’ AHL affiliate in Bridgeport for Thursday’s practice.

“It’s good for him,” Lambert said. “He sees our shooters, which is different for him than being down there and seeing the same shooters every day. Then he comes in and he gets a little feel for the day after a game where we didn’t play very well.”

Appleby, 28, an undrafted free agent, is 5-4-0 with a 2.86 goals-against average and an .898 save percentage for Bridgeport. Jakub Skarek, 24, a third-round pick in 2018, is 3-13-2 with a 3.51 GAA and .877 save percentage for Bridgeport.

Tristan Lennox, 21, a third-round pick in 2021, and Henrik Tikkanen, 23, a seventh-round pick in 2020, comprise the goalie tandem for the Islanders’ ECHL affiliate in Worcester. Lennox is 5-6-2 with a 2.94 GAA and .909 save percentage. The 6-7 Tikkanen is 6-5-2 with a 3.11 GAA and .896 save percentage.

So none of the goalie prospects — granted, that term doesn’t really apply to Appleby — are posting consistent numbers.

The previous three seasons, the Islanders had former Devils No. 1 netminder Cory Schneider — now retired and working as a television analyst — as the in-case-of-emergency-break-glass option in Bridgeport.

The Islanders have been fortunate because Sorokin and Varlamov — who have proved to be excellent No. 1 goalies — have been durable. Schneider was called upon to play only one NHL game in his three seasons in the organization.

“Well, I know one thing,” president/general manager Lou Lamoriello said before training camp opened in September. “When I go to bed at night, I feel pretty good knowing our goaltending.”

He was talking about the NHL roster.

Varlamov’s absence on Thursday demonstrates that Lamoriello might have to acquire a goalie with more NHL experience to bolster Plan B in net.

Notes & quotes: Casey Cizikas missed Thursday’s practice because of illness. Lambert said he expects the fourth-line center to be available against the Capitals . . . Defenseman Scott Mayfield (injured reserve/upper body) continued to skate with the team, but Lambert said he will not play on Friday. It will be the eighth game Mayfield misses . . . Defenseman Grant Hutton was re-assigned to Bridgeport to create a roster spot for Appleby.

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