While Pep Guardiola's City will play the same number of games in a short space of time, they get an extra day to recover from their recent win over Bournemouth, as they don't take on Newcastle United until Wednesday.
The Etihad Stadium side already have a 13-point lead over the Red Devils before Tuesday's game, but Mourinho believes they have an even bigger edge thanks to their longer recovery periods.
"I'm not just talking about the privileged runs with six days to recover and enjoy and to prepare but ones with a little bit more than us," Mourinho told Sky Sports.
"For example the ones with a little bit more rest, like one day. The extra day means a lot because it means they aren't working [on Christmas Day], means they can be back to work and be ready for the next day.
"The problem I see here is the difference between some teams."
United have slipped up in three of their games this month, having lost to CSKA Moscow in the Champions League and Bristol City in the Carabao Cup before Leicester upset them with a late equaliser on Saturday.
But Mourinho feels his side have been creating enough chances to win games, and hopes to see them kill off their next opponents like they should have done against the Foxes.
"I hope we play so well as we did against Leicester because in 10 matches we play so well and create so many chances as we did, in nine matches we would win three or 4-0," Mourinho added.
"In one match what happened was what we call unpredictable football.
"That happened to us but I don't think it happens many times when we play so well. In this case we played so well and so creative and create so many chances, but of course in football you have to score the goals and take the chances."