The Wales international was substituted after an hour when the La Liga giants last met, but the Italian believes that the world's most expensive player has now adapted to the Spanish game.
"In the first game he was switching positions with Angel Di Maria at times, but now he is used to playing on the wing and the team is balanced so we won't change that," Sky Sports News quotes Ancelotti as saying.
"He is in good shape. His physical condition is very important for him and I am sure he will play a great game. We haven't done any special work with him to improve his fitness or speed. He just has great genes and has done the same work as everyone else.
"He is in really good condition and we are very happy with him. He had some problems at the start of the season but now he is looking good and we hope that will continue until the end of the season."
Real head into the match as league leaders, with Barcelona four points behind them in third.