The Turkey international made an impressive first appearance at Aranda de Duero, nearly six months after his move to Santiago Bernabeu from Super Lig club Fenerbahce. Guler suffered a series of injuries since penning a six-year deal for a reported transfer fee of 20 million euros.
The LaLiga leaders cruised into the last 16 with the win against Arandina.
"Guler performed well for an hour. It was important for him to get used to playing with the team. He will gradually build up his fitness level and intensity," Ancelotti told reporters.
"We have to be patient with him, but he showed his quality in the first half. It's important that he's back.
"He has personality and character, and that's good. He has good quality with his left foot, but character is an important aspect to do well at Real Madrid."
Madrid next host rivals Atletico Madrid in the semi-final of the Spanish Super Cup on Wednesday.
