Brady Tkachuk of the Senators scores a second-period goal against Rangers...

Brady Tkachuk of the Senators scores a second-period goal against Rangers goalie Alexander Georgiev at Madison Square Garden on Monday. Credit: David L. Pokress/David L. Pokress

That’s the thing with young teams, and it’s something Rangers coach David Quinn has said over and over: They’re going to be up and down.

And after a Saturday in Nashville in which the Rangers left the arena feeling really, really good about themselves, things didn’t go nearly as well Monday night in New York.

Playing against the lowly Ottawa Senators, who entered the game with the fewest points in the 31-team NHL, the Rangers were simply dreadful, dropping an ugly 6-2 decision that snapped their winning streak at two games and dropped their record to 5-6-1. Ottawa is 4-8-1.

“You could feel it on the bench after we made it 1-0,’’ Quinn said. “I think we thought it was going to be easy. And really, the game was two different games. It was the 11-minute game and it was a 49-minute game. And unfortunately, we played the 11-minute game instead of the 49-minute game.’’

The Rangers actually won the 11-minute game. They were leading 1-0 on a goal by rookie Kaapo Kakko and were outshooting the Senators 11-1. Then things changed dramatically. The Senators scored two goals before the first period was over and were outshooting the Rangers 14-11 by the time the horn sounded to end the period.

“The first 10 minutes, we played the way we were playing the last two games, and then suddenly they scored a goal and we get back to the way we were playing, when we were on the losing side,’’ Jesper Fast said. “I mean, it’s frustrating. ’’

Playing their third straight game without No. 1 center Mika Zibanejad — who skated in the morning — the Rangers allowed the Senators to score five straight goals from the 14:56 mark of the first period to the 14:56 mark of the second.

Former Ranger Vladislav Namestnikov, who was traded to the Senators on Oct. 7, two days after the Rangers won in Ottawa in the second game of the season, added insult to injury when he scored the Senators’ last goal on a power play early in the third.

Rangers goaltender Alexandar Georgiev, making his third straight start, wasn’t as sharp as he had been in his previous two starts, allowing six goals on 30 shots.

“It wasn’t his best game,’’ Quinn said.

Kakko’s third goal of the season (assisted by fellow rookie Adam Fox) gave the Rangers the lead at 8:47 of the first period. Then a fight broke out between Brendan Lemieux and Ottawa’s Mark Borowiecki. It was the second fight of the game, after the Rangers’ Tony DeAngelo tangled with J.C. Beaudin earlier in the period, and it seemed to change the momentum.

“It could have,’’ Quinn said when asked if he thought the fight might have turned things around. “Fighting can do that. I wish it would have inspired us a little bit more. But it didn’t.’’

Jean-Gabriel Pageau scored the first of two goals to tie the score at 1-1 at 14:56 of the first, and after another altercation — Ryan Strome wanted to fight Brady Tkachuk but the officials broke it up — Strome and Anthony DuClair got roughing penalties and Jacob Trouba got a cross-checking penalty, giving Ottawa a power play at 17:17.

Thirty-nine seconds later, Brady Skjei was sent off for tripping, giving the Senators a five-on-three, and 35 seconds after that, Tyler Ennis scored to give Ottawa a 2-1 lead.

Ron Hainsey, Pageau (on a breakaway) and Tkachuk scored in the second to give Ottawa a 5-1 lead before Artemi Panarin’s power-play goal made it 5-2.

Namestnikov scored on the power play at 5:35 of the third to make it 6-2. It was his fifth goal of the season, which is more than any Ranger except Panarin, who has six.

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