The 100 million-euro Wales winger, who was sidelined by a thigh strain last month, also featured as a second-half substitute against Malaga at the weekend and Ancelotti said he was now fit enough to be included in the starting line-up.
The Real fans showed they are impatient to see Bale in action by giving him a rousing cheer when he entered the fray on Wednesday but while he looked lively he made little impact with the game already won and the Italian champions down to 10 men following the dismissal of Giorgio Chiellini.
"He came on when the match has lost its intensity," Ancelotti said.
"That didn't help him play better but it was important for him to get some time on the pitch," added the Italian. "I think now he is ready to start a match."
Leaders Barca dropped their first points of the campaign in a 0-0 draw at Osasuna on Saturday and stuttered in Europe on Tuesday when they had to come from a goal down to rescue a 1-1 draw at AC Milan.
They have a three-point lead over third-placed Real after nine matches but Real will be the more confident side going into the 'Clasico' after two wins in five days and given the superb form of Cristiano Ronaldo.
The Portuguese forward scored both goals against Juve, taking his tally in the Champions League to seven in three matches this season.