Moments after Niedermayer was hit on the back of the head by a slap shot off the stick of Ottawa defenseman Anton Volchenkov, Van Allen ended a 54-game postseason scoring drought.
Skating through the left faceoff circle, Van Allen worked a give-and-go with Martin Havlat. He got the return feed just above the crease and redirected the puck past helpless goaltender Martin Brodeur to make the Senators' first conference finals game a successful one.
"I am still tingling all over," said Van Allen, who is in his second stint with the Senators. "As a kid growing up in the streets in Saskatchewan, you always dream of scoring the overtime winner. You just hope for the chance that you get one. It was an unbelievable feeling."
Top-seeded Ottawa squandered an early two-goal lead but maintained home-ice advantage. The Senators also broke an NHL playoff record by holding their 11th consecutive opponent to two goals or fewer.
Joe Nieuwendyk and Jay Pandolfo scored in the second period for the Devils, who are trailing in a series for the first time in this year's playoffs. They were playing for the first time since eliminating Tampa Bay eight days ago.
"Our first goal was to win one (road) game," Brodeur said. "This was just a little bump in the road. We'll try our best in the next one. If it doesn't happen, there are still more games."
Niedermayer appeared dazed behind the bench before he was escorted to the locker room. His status for Game 2 on Tuesday is uncertain.
"I haven't talked to him yet," Devils Coach Pat Burns said. "I know that we missed him. We were a little concerned. He was a little woozy when he was coming off the ice."
In Game 3 of the conference semifinals at Tampa Bay, New Jersey defenseman Scott Stevens was hit above his left ear by a slap shot early in the first period. Without their captain, the Devils fell behind, 3-0.
"It was a much different scenario," Nieuwendyk said. "Scott went down in the first shift while tonight, we didn't have time to think about it. Niedermayer is our best two-way defenseman and we hope he'll be OK."
But the Senators scored on consecutive shots in a 73-second span. They took the lead at 6:10 as Magnus Arvedson's turnaround shot from the edge of the right circle hit the skate of teammate Chris Neil and deflected past Brodeur.
It was the first career playoff goal for Neil, who had six in 68 games during the season.
"We know we've got to go to the net and create traffic and that's how we get goals," he said. "It wasn't a pretty one, but I'll take it."
Just over a minute later, Brodeur made a right pad stop on defenseman Chris Phillips' blast from the left point, but Todd White got the rebound at the bottom of the left circle and flicked it past the diving goalie.
"It was a great point shot by Chris Phillips," White said. "I was able to spin off my check and was fortunate the puck came over to me."
Brodeur regrouped and helped keep the Devils within two goals. He got his glove on a wrist shot by Bryan Smolinski off a three on one with three minutes to go in the period, denied Phillips on a point-blank rebound early in the second and dived to deny Radek Bonk off a four on one just over three minutes later.
Brodeur also got his right skate on Smolinski's wrister from the slot with 14:14 left in the second and made a diving pokecheck on Daniel Alfredsson with 12 minutes to go.
"He had a rough start, probably due to not playing in two weeks," New Jersey defenseman Brian Rafalski said. "Ottawa played a wide-open game and Marty kept us in it."
The momentum gradually swung to the Devils, who got on the board at 14:19 on Nieuwendyk's first goal in nine games. Sprung by a pass from Jeff Friesen, Nieuwendyk broke down the left side and squeezed a shot between Lalime and the near goalpost.
Just 2:32 later, Pandolfo got a pass off the left boards from Jamie Langenbrunner and ripped a one-timer from high in the left circle over Lalime's glove for his second playoff goal.
"I think our experience showed in the second half of the game, where we didn't get excited, everybody stayed calm, and we talked about it after the first period," Burns said. "I knew it was going to be tough in the first 10 minutes of the game."
Ottawa Coach Jacques Martin used his timeout after the goal and tried to steady his team during the second intermission.
"We weren't good at that time, we made some mistakes and we talked about a couple of things we had to be better in," Martin said. "We had 20 minutes to go out and work hard. Opportunities are going to come if you do the little things right."
Brodeur again came up big with 15 1/2 minutes left in regulation when he used his left pad and glove to stop Havlat on a breakaway. He finished with 27 saves while Lalime stopped 32 shots.