"We have to refocus and move on."
The Islanders had three shorthanded breakaways in the second period.
"We were a little too casual, and whenever one of our point men were under stress, there was nobody there to back them up," center Danny Briere said. "It's something we'll have to talk about and resolve."
The Flyers got to within 3-1 - and woke up the crowd - when Matt Read tipped in Sean Couturier's shot with 9 minutes, 12 seconds left. Less than three minutes later, Islanders goalie Evgeni Nabokov (40 saves) robbed Wayne Simmonds out front.
The Flyers, coming off a 5-1 conquest of Minnesota, had 41 shots, but most were from the perimeter.
"We shot a lot of pucks, [but] not from quality spots," said Read, who leads NHL rookies with 14 goals. "We were on our heels pretty much the whole game, and it was unacceptable."
Grabner's empty-net goal came with 0.8 second remaining.
The Flyers, who had few chances on their four power plays and committed numerous turnovers, have won just 11 of 20 games at home (11-7-2). They are a league-best 16-7-2 on the road.
Couturier had his five-game goal-scoring streak snapped.
Goalie Sergei Bobrovsky stopped four breakaways and kept the Flyers within striking distance.
"If it wasn't for Bob tonight, this game wasn't even close," said Claude Giroux, who has just one goal in his last 12 games.
The Islanders took a 1-0 lead when Moulson scored after being left alone in the slot 28 seconds into the second period. Moulson was set up by the ever-present John Tavares (two assists), who increased his point streak to 11 straight games.
Bobrovsky, now 7-1 in his career against the Islanders, stopped Grabner on a shorthanded breakaway early in the second period. The Flyers got another power play about five minutes later, but again it was the Islanders who had the best scoring chance. Again, Bobrovsky stood tall, denying Tavares as he was ahead of the pack.
Giroux, the Flyers' all-star center, tried to energize his team by getting involved in a (fairly even) fight with Dylan Reese with 2:19 left in the second period.
It didn't work.
Timonen said Giroux has been "maybe putting too much pressure on himself" during his scoring slump. "You can't play 82 games and be the best player out there every game," Timonen said. "You're going to have ups and downs, and he's going to be fine."
Giroux was minus-3, and he is minus-9 in his last six games.
Contact staff writer Sam Carchidi at firstname.lastname@example.org, 215-854-5181, or @BroadStBull on Twitter.