Ancelotti blasted his side for their poor performance as Rennes punished them at the Parc des Princes.
Rennes, who had been reduced to nine men, pulled off a huge shock when Romain Alessandrini scored a fantastic volley in the 13th minute and did not crumble when goalkeeper Benoit Costil was sent off after Nene's leveller.
Julien Feret scored Rennes' winner from a free-kick as the Breton side held PSG back even after Jean Makoun's second-half dismissal.
Ancelotti knows his side quickly need to raise their game ahead of Wednesday night's Champions League clash away to Dynamo Kiev.
"It is unbelievable. Our attitude out there was not good," Ancelotti told the official Ligue 1 website.
"We have just lost two games at home and that is unacceptable for a team that wants to win the league. We are yet playing as a team and we are going through a bad spell.
"We have to change things and I am going to change things. We need to be more concentrated, more determined and harder. It is not right.
"We need speed and intensity. I am not scared to say it; we are in a crisis.
"We have to emerge from it quickly. If we show character and professionalism we will pull through."