The Black Cats remain five points above the relegation zone following their loss at Villa Park.
However, they could take a huge step towards survival when they welcome Stoke City to the Stadium of Light on Monday.
"The manager was angry [after the Villa game]," Sky Sports News quotes N'Diaye as saying. "He said that we need to work more. We need to work more this week to beat Stoke, and he said that to us.
"It is not normal to lose by five goals, it is not normal to lose six goals in one game. It was a bad day. It was a very bad day for us, and I do not know why.
"You can't concede six goals in one game, it is just not normal. We are back in the relegation fight."
Wigan Athletic have a game in hand on Sunderland.