Los Blancos made it two from two in La Liga this term with the victory, with Modric along with Karim Benzema, Vinicius Junior and Federico Valverde among the goals.
But Modric’s 41st minute strike restored Real’s lead after Iago Aspas had squared the game up with a 23rd minute penalty.
“Modric is immortal,” Ancelotti said.
“He’s clever, well prepared and is playing really well. His goal changed the game, because until the goal it was an even match and it was really competitive.
“Celta pressed in the first half. They pressured us well and we had difficulty in possession.
“In the second half they lowered the intensity and we worked the ball well out from the back and the transitions were spectacular.”
Modric’s performance came amid the move of Brazilian midfielder Casemiro to Manchester United.
Casemiro’s exit has opened the door for Aurelien Tchouameni to step up in the Real midfield.
“Tchouameni has different qualities to Casemiro,” Ancelotti said. “He played really well today and hes shown what hes been doing so well in training.
“He defended well and he enjoyed playing his part in the teams performance.
“He looked comfortable because Modric, Camavinga and he did a great job, along with the help of Valverde, who always does an important job defensively.”