The Portuguese went off as a precaution at half time during the first leg at the Bernabeu on Tuesday suffering a hamstring strain during the 1-1 draw.
"He is available to play and will be able to start the game. All the players are ready but that doesn't mean I won't make changes," Ancelotti told a news conference in Madrid.
There were initial fears that Ronaldo had suffered a recurrence of the knee injury that he suffered at the back end of last season and carried into the World Cup but that was quickly played down.
Real have set their sights on winning all six trophies on offer this season having won the European Super Cup against Sevilla, but they were lacklustre against Atletico and the draw has put their city rivals in the driving seat.
"We are used to playing against Atletico and they are used to facing us. I don't think the style of game will be different, they now have the advantage from their positive result at the Bernabeu but I think it will be the same type of match," said Ancelotti.
"We need to defend, counter attack well and take advantage of free-kicks."
Angel Di Maria has been strongly linked with a move away from Real and despite his good form started on the bench in the first leg, but Ancelotti says he will treat him as any other member of the squad if he ends up staying.
"Di Maria asked to leave this summer and rejected the option to renew. We will look for the best option for him and if at the end of August he is still here then he will work with us like last season," he said.