Those involved in this summer's competition have been given extended breaks while the rest of their teammates begin pre-season, leaving United and a host of other teams without a number of key players in the buildup to the new campaign.
The Red Devils are currently on a pre-season tour of the United States, but Mourinho has suggested that their World Cup absentees will leave them struggling against Leicester City and Brighton & Hove Albion in the first two games of the Premier League season.
"I've got the facilities as always, I am happy with the matches, the evolution of the matches, but I don't have the players to work. I don't have the majority of the players that are going to be in the squad on August 9 when the market closes and we can make the squad official for the season, so of course I'm not happy to have just a few players," Mourinho told reporters.
"If you look to the players that Chelsea and Liverpool have in the pre-season, especially these two teams, also Arsenal, if you compare, you see the difference of the situation. Manchester City and Tottenham are in a similar situation as we are, but you can see that some other clubs are in a different situation.
"The first match of the season is against Leicester and the second one is against Brighton. I look to Leicester, if I'm not wrong, and only [Harry] Maguire and [Jamie] Vardy were in the World Cup in the final phase, because [Kasper] Schmeichel was back sooner. So I think a team working for six weeks minimum with all the players except one defender and one striker that can join in the last weekend and probably be available, it's clear it's a much better situation for them.
"When I look to Brighton, I don't think they have any player that isn't doing a very good pre-season with lots of training and matches. So against Leicester and Brighton the situation is not amazing for us. I would say the third match, playing against a team in a similar situation as us, after three weeks is okay. It's almost like you need a pre-season with them, but for the first two matches we have to fight for the points, so that is what we are going to do with the players available."
United's next pre-season match comes against AC Milan in Pasadena in the early hours of Thursday morning.