Castellanos now has 11 goals in 29 appearances in La Liga this season.
Ancelotti had said his team needed to stay on positive winning momentum in the league to remain at the top of their game in Europe, but was miserably off the ball at Girona.
“A tough night, yes, when you lose a game like this it is a tough night,” said the Italian.
“On a defensive level it was a poor game, and that was key to the match … The individual level was also below normal.
“Everyone had it, not just some people. The level was very low, because the defensive aspect, which was very good in recent games, is what we have forgotten today.”