Pittsburgh is tied for first place in the eight-team MassMutual East Division with the Washington Capitals, who defeated the New York Rangers 6-3 on Monday. The Penguins, Capitals, Boston Bruins and New York Islanders have clinched the four Stanley Cup Playoff berths from the division.
"I just don't think we were on our game," Sullivan said. "I don't think we were skating, I don't think we were playing the game that we've played here for a long time that's brought us success."
The teams will play again here Tuesday.
Kevin Hayes scored for the first time in 17 games to give the Flyers a 1-0 lead at 5:58 of the first period. He took a shot from near the right post that DeSmith appeared to sprawl and save with his glove, but video review showed the puck crossed the goal line.
It was the first time in 11 games that Philadelphia scored the first goal.
"When you get that first goal, it feels like it kind of energizes the team, the bench a bit, and it takes a lot less energy, or at least it feels that way, to play the right way when you have a little bit of a lead like that, versus trying to chase down games, which is what we've kind of gotten in the habit of doing way too much this year," Flyers forward James van Riemsdyk said. "Definitely nice to kind of start like that and keep going from there."
Shayne Gostisbehere made it 2-0 with a power-play goal at 14:08, and Giroux scored to make it 3-0 at 15:12.
Wade Allison put the Flyers ahead 4-0 at 7:50 of the second period.
Crosby scored on the power play to make it 4-1 at 2:30 of the third period.