The loss snaps a three-game winning streak for the Sabres, who had a chance to move within six points of third place in the Atlantic Division with Toronto's loss in Pittsburgh.
The Senators fed off the momentum of Namestnikov's goal, tying the game on a shot from in tight by Artem Anisimov less than two minutes later. Drake Batherson got in behind the Sabres defense and buried his own rebound on the very next shift, then Anisimov added a second goal on a power play.
"We just lost everything that we had built during the first 10 minutes," Krueger said. "We broke down in the D-zone like we haven't been lately. We stood and watched a team that went hungry on their toes and felt the opportunity and they used it."
Both times the Sabres cut the deficit to one, on goals from Kyle Okposo and Victor Olofsson, the Senators restored their two-goal cushion on goals from Jean-Gabriel Pageau. The Senators forward concluded his season series agains the Sabres with five goals in four contests.
Buffalo's biggest push came trailing 5-4 at the start of the third period, but Ottawa goalie Craig Anderson shut the door. Anthony Duclair put the finishing touches on the victory with an empty-net goal in the final minute.
The Sabres have three days between games, then play a back-to-back set that begins Saturday in Pittsburgh and concludes Sunday at home against Winnipeg.
"I think everybody's got to realize where we're at," Montour said. "We let another one slip away. After the bye week there, we've let a number of games slip away and those are big points. Hopefully everybody in this room realizes we let another one go there and we've got to get back to work tomorrow."
"We've got to win games," added Okposo. "That's it. That's the bottom line. We have to trust the way that we play is going to produce the results and we just fell asleep for a little bit, and it that cost us. We can't have that. we can't have those mental errors right now."