The coach believes he made the wrong choice in his first Clasico and predicts Manchester City will struggle when it makes the return trip to Camp Nou in the Champions League.
With Xabi Alonso and Asier Illarramendi injured, the Italian chose to play the defender further up the pitch than usual and the Blancos were beaten 2-1 at Camp Nou thanks to goals from Alexis Sanchez and Neymar.
The former Chelsea and Paris Saint-Germain coach acknowledged that the Clasico was a bad time to put Ramos in an unfamiliar position, despite being short of options in central midfield.
"It was a mistake, yes," Ancelotti told Goal. "But you have to remember that in that game had no Xabi Alonso and Asier Illarramendi was not fit."
Madrid's rival was in action in the Champions League on Tuesday, defeating Manchester City 2-0 in the last 16 first leg, and Ancelotti thinks that it will take a superhuman effort to beat Gerardo Martino's men at Camp Nou.
"They played a very intense game at the Etihad Stadium," the coach said of the Blaugrana's victory in midweek, "and it will not be easy for Manuel Pellegrini's men at Barcelona.
Real Madrid plays its last-16 first leg tie at Schalke next week before the second leg at the Santiago Bernabeu in March.