Pellegrini said: "We picked up a good result at a ground that has not been good for us in the past and this has allowed us to remain as leaders.
"After the break we lowered the intensity due to the derby (against Atletico) and to avoid cards."
Casillas chased the ball outside his area after failing to grab Jaime Gavilan's cross, and Parejo slid in to fairly rob the Spain goalkeeper of possession.
The ball was picked up by Leon, who fed Parejo on the edge of the area and he fired into an unguarded net.
"I don't think any Getafe player will be proud of this action. I hope that my team won't do likewise in the same situation," Pellegrini added in AS.