United fought back for a 2-1 victory against the Italian champions in Turin on Wednesday while City cruised through a home game against Ukrainians Shakhtar Donetsk 6-0 and the Portuguese manager was quick to point out that context.
He said the players were feeling it physically and mentally.
Mourinho was without French midfielder Paul Pogba, sidelined with injury, and while the United boss did not want to use that absence as an excuse, he said the need to play Marouane Fellaini had limited his later options.
Despite City largely controlling the game, it took a late third goal by their substitute Ilkay Gundogan to ensure there could be no repeat of United's dramatic comeback to win 3-2 in the same fixture last season.
United remain eighth, 12 points behind the leaders, and manager Mourinho refused to talk about his team still having any sort of title chances.
Pep Guardiola, City's manager, was quick to point out that Liverpool and Chelsea had also made strong starts to the campaign and said it was too early to be discussing where the title may be heading.
The Manchester United manager said the players were feeling it physically and mentally.