Paolo Di Canio believes complacency was the reason for Sunderland's 6-1 defeat against Aston Villa on Monday night.
Having gone into the Villa game as favourites after back-to-back league wins over Newcastle and Everton, Sunderland were humiliated by a result that thrust them back into the relegation fight.
Di Canio has said his players, having seen both Wigan and Newcastle fail to secure victories in their games, were not fully focused on their own task at Villa Park.
"Complacency was one of the reasons, to add to others," Di Canio told the Sunderland Echo. "We had two wins in a row and were big heroes in the city. A poison developed, albeit a positive one, after two wins in a row. Then, after the weekend results for Wigan and Newcastle, we knew we were up if we won the game.
"In the way we analysed the game on Tuesday, there was an agreement that we relaxed too much after seeing the results for Wigan and Newcastle. The results were good for us in a way, but they were a poison for the performance at Villa Park.
"You can see that we received a strong slap from Aston Villa. We have to make sure that doesn't happen again, otherwise we will remain in trouble."
He also said his players had been left fatigued as injuries had left him unable to rotate his squad.
"The players we've used have mainly been the same," he said. "That looks like an excuse, but it's not, because I knew before I took this team that we didn't have many players. But there wasn't a habit to win two games in a row, after nine games without winning, including Chelsea.
"Then to win at Newcastle after 13 years, the players know about the club's history, especially the ones who live around here. And then after the Everton win, we had nine days to think about things. We'd spent a lot of physical and mental energy and relaxed 20%, but now we're okay."