The Black Cats were hammered 6-1 by Aston Villa after they suffered a second-half collapse that was as unexpected as it was sudden.
After two wins under new boss Paolo di Canio, it brought their forward momentum to a shuddering halt and still leaves them needing one more win to assure themselves of top flight football next season.
And N'Diaye has never known anything like it.
"I have never lost by that many in my life," he said.
"It was a very bad for us and I don't know why.
"Before the game we had good motivation to take three points. Instead we conceded six goals. It is not good enough.
"You can't concede six goals in one game. It is just not normal."
Now Sunderland are one of three clubs who have a five point gap on third-bottom Wigan, although that could be reduced to worrying proportions if the Latics win their game in hand against Swansea next week.
And N'Diaye accepts there is work to do.
"Of course we are back in the relegation fight," he said.
"I thought before the game that we only needed one win and I still think that.
"If we won on Monday we would have had 40 points. Now we don't have a choice.
"We need to win against Stoke next Monday. If we do that we will still be safe."
N'Diaye confirmed Di Canio was not in a good mood afterwards, although admitted in such circumstances it was "normal" for the fiery Italian to be angry.
Di Canio himself was more calm about the situation, insisting that whilst the manner and size of the defeat hurt, it should not obscure the progress that has been made in the month since his arrival.
"We can't cancel what we did in the last two weeks, otherwise we are stupid," he said.
"Although I didn't want us to go down in this game, I was curious to see how we reacted. We never went down.
"Straightaway we showed good character and scored a very good goal.
"But Aston Villa are the best side in the league on the counter-attack, so their second goal was not an accident."
Although Sunderland's remaining three fixtures are the least arduous of those around the drop zone, and their goal difference remains another advantage, they cannot afford to ease off just yet.
To that end, a successful appeal of Stephane Sessegnon's red card, which threatens to bring his campaign to an early conclusion, would be an advantage, even if it is not particularly likely.
"That was a blow for us because I don't think it was red," said N'Diaye.
"He is not a bad player for tackles. It is the first foul I have seen from him
"If he is out for three games, that is very bad.
"But we do have other players who can step in, make chances and score goals though.
"If Stephane can't play it is up to us to start doing that and showing that we don't rely on him."
More goals like the one Danny Rose scored would be handy, although the England Under-21's first this season looks like being his last for Sunderland.
Tottenham boss Andre Villas-Boas has apparently indicated Rose will be part of his plans for next season and therefore he will not be returning to the Stadium of Light.