The Black Cats were thrashed by Villa on Monday night to plunge themselves right back into the relegation mire.
N'Diaye said: "The manager was angry. This is normal because we lost six-one. It is not normal lose by six. He said that we need to work more. We need to work more this week to beat Stoke and he said that to us.
"Of course we are back in the relegation fight. I thought before the game that we only needed one win and I still think that.
"If we had won on Monday we would have had 40 points: that's OK. We have lost so we still need a win and we need to win against Stoke. If we do that then we will still be safe.
"It was a bad day. It was a very bad for us and I don't know why. Before the game we had good motivation to take three points and now we have conceded six goals. It is not good enough. You can't concede six goals in one game. It is just not normal."