City meet Barcelona needing to overcome a 2-0 deficit to progress in the Champions League.
Cousillas a 56-year-old former goalkeeper, first worked with Pellegrini at former club San Lorenzo. He then followed him to River Plate, Villarreal, Real Madrid and Malaga before coming to the Etihad Stadium.
He revealed that he had stood in once for Pellegrini in the past and appeared to be relishing the opportunity.
"Officially only once in Spain," he said. "I have had very few chances to do that."
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