The Flyers have lost four straight, and they have dropped six of their last 14 games against non-playoff teams, such as Ottawa.
"It's very alarming. We took some penalties tonight, and we just didn't do the little things Ottawa did," veteran defenseman Sean O'Donnell said. "We had a 2-1 lead, and then we were just flat. I can't explain it. With first place there - we were right there with Washington - to come up with an effort like that is just inexcusable."
"It's tough to come into a building, where we desperately needed points, and just not play well," captain Mike Richards said after scoring two goals, both on the power play.
Meszaros, whose team has lost seven straight in Ottawa, was asked how much the Flyers miss injured defenseman Chris Pronger.
"We miss him, obviously, and Danny [Briere], too. They're a big part of our team, but we have to accept that and play without them," Meszaros said. "I don't know if they're going to be ready for the playoffs. If they're not, we have to play even harder than we're playing. We just can't play like we did tonight, that's for sure."
Briere and Pronger are expected to be ready for next week's playoffs. Briere could return Friday in Buffalo.
Richards had his fourth two-goal game of the season, with three of them against the Senators.
Richards blamed the team's slump on "a lack of intensity and a lack of focus for these games, led by me. It has to change, and it's going to change."
Ottawa took a 4-2 lead when Jason Spezza (four points) scored the Senators' third power-play goal of the night, firing a left-circle shot into the net with 11 minutes, 58 seconds left.
Milan Michalek added an empty-net goal with 1:18 remaining.
The Flyers, missing injured penalty-killing leader Blair Betts, allowed three power-play goals for the third time this season - the first since Nov. 20 against Washington.
Now the Flyers will try to regroup. They will play in Buffalo on Friday, then close the regular season by hosting the Islanders on Saturday.
If the Flyers were struggling in the middle of the season, it would be one thing. But to play listlessly in the home stretch and "come out and lay an egg like tonight" is puzzling, O'Donnell said. "We need to look in the mirror and get ready to go Friday night."
With Danny Syvret in the penalty box for holding, Ottawa's Ryan Shannon tied the score, 2-2, with a point drive that got past Sergei Bobrovsky, who was screened by Spezza, with 1:18 left in the second period.
Just 1:04 later, Bobby Butler scored on a breakaway, putting a shot between Bobrovsky's legs and giving the Senators a 3-2 lead.
Flyers coach Peter Laviolette said that his team had "no jump" and that he found it "extremely alarming" for what was at stake.
"It's not good enough. At all. By any stretch," added Laviolette, whose team is 6-8-5 since a 4-1 loss in Ottawa on Feb. 26.
Contact staff writer Sam Carchidi at firstname.lastname@example.org. Follow on Twitter at www.twitter.com/BroadStBull.