City take on the Catalan giants at the Etihad Stadium on Tuesday evening buoyed by Saturday's FA Cup triumph over Mourinho's Chelsea.
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Mourinho, however - who has a long and fractious history with Barcelona - believes Tata Martino's men have slipped from their pedestal enough this season to give City a good chance.
Mourinho told ITV: "Of course by history Barcelona is the favourite, but this Barcelona, this season, is showing that it is not the same in previous years.
"Of course they have Messi, he is special, and they have more than him. But I think this is the worst Barcelona of many many years, so City have a chance."
Mourinho is widely despised in Catalonia for defeating Barcelona with Chelsea and Inter, and also for his treatment of Pep Guardiola and Tito Vilanova while Real Madrid manager.
He has also made headlines since returning to Stamford Bridge with a series of perceived slights towards City and Pellegrini, and last week referred to Arsenal boss Arsene Wenger as a "specialist in failure".