The Manchester team travel to Hull City having won the first leg of their semi-final clash 2-0.
Looking at the current fixture list, they are due to play City on February 26, which is the date of the EFL Cup final at Wembley.
United are the only team still in four competitions and Mourinho is afraid this will reduce options for a new date for their trip to the Etihad.
When asked if United could challenge for all four trophies, Mourinho said, “I don't know, but I know it is going to be hard, if we go step-by-step, it is going to be hard.
“If we go to the fields of the 'ifs,' if we go to the EFL final and don't play Man City, I am sure they will choose an amazingly difficult moment for us to make us play that game.
“Then we have the Europa League, which is non-stop. The Champions League have a last 16, but we have a last 32 and a last 16, so if we progress, it will be really difficult.”
United would face a minimum of 66 games, if they progressed to the finals of the EFL Cup, FA Cup and Europa League and Mourinho revealed he is already planning for such scenario.
“We go competition by competition, if we lose we go out, if we draw in the FA Cup, we have another match,” he said.
“It will be difficult.
“I have a board on my wall with all the possible matches and the image is quite difficult.
“Some teams have a wonderful life, they play at the weekend, have two days off and everyone rests.
“Then they have three to five days to play next match, with ideas clear, clear tactical work, every detail and that is a big difference.
“So let's see what happens.” The coach said.