Even when the Islanders take the day off, as they did Sunday, the hits keep on coming.

Dennis Seidenberg suffered a broken jaw Saturday against the Panthers, according to two sources, and the Islanders will find out Monday how long they will be without another regular defenseman.

Seidenberg, 35, signed a one-year, $1-million contract on Sept. 28, well after the start of training camp and toward the end of a successful run for Team Europe at the World Cup. The Bruins had bought him out of the final year of his contract in June.

Seidenberg has been surprisingly effective and productive as an Islander, leading the team’s defensemen with four goals and eight points through 14 games. It’s believed he suffered the broken jaw when he blocked Michael Matheson’s point shot in the third period.

The puck ricocheted to Kyle Rau, who beat Jaroslav Halak to start Florida’s comeback from a 2-0 deficit. The Islanders lost in overtime, 3-2, a crushing defeat that was made all the more painful with the Seidenberg news.

With only six healthy defensemen on the roster, the Islanders likely will summon Scott Mayfield from Bridgeport to step into the lineup for Monday night’s game against the Lightning at Barclays Center. Tampa Bay has outscored the Islanders 10-2 while beating them twice in the last 12 days.

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Travis Hamonic (broken thumb) still is a month away from returning and Ryan Pulock (broken foot) has 10 to 14 days left in his recovery. Seidenberg is the third right-side defenseman to be lost to injury, and the Isles’ defensive depth has been stretched to its limit.

Add in their abject frustration after losing more points late in a third period, and the Isles are trying to save their season and regain their confidence. “We have to stay together. That’s the only way we can break out of this,” John Tavares said. “There’s still a lot of hockey left.”