Mourinho has rounded on the club's hierarchy for failing to sufficiently strengthen in his opinion despite the signings of Brazilian worldwide midfielder Fred and Portuguese full-back Diogo Dalot.
Fabinho and Naby Keita will add extra dynamism and depth to the midfield, while Xherdan Shaqiri offers better back-up to the prolific front three of Sadio Mane, Roberto Firmino and Mohamed Salah.
"The other clubs who compete with us are really strong and already have fantastic teams".
Romelu Lukaku, Marcus Rashford and Phil Jones all came back early from their own extended breaks but Mourinho sounded downbeat again when he was asked about the new season.
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But United are even more shorthanded by the quick turnaround between the World Cup and the start of the new season. Sometimes its also hard to believe I won eight championships and three Premier Leagues, and I keep thinking the second position previous year was one of my biggest achievements in England'.
Jose Mourinho has made no secret of his disappointment at Man Utd's transfer business so far as they've only signed three players with midfielder Fred the only one considered to be a first team regular.
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I think it'll be tough but if they were to go of the Champions League and they had free weeks after Christmas, Saturday-to-Saturday they could be a real threat this season to City. "Nobody is consistent with 11 players".
World Cup victor Paul Pogba is also unlikely to be fit for the game at Old Trafford.
"The players with a proper pre-season are fit and they are strong".
The experienced Maurizio Sarri is the latest incumbent in the Chelsea hot seat having replaced Antonio Conte last month.
Unlike most of Mourinho's stars, Sanchez had a first full summer off in nine years as Chile failed to qualify for the World Cup, ensuring he should be fresh enough to show why United handed him a contract reportedly worth a potential £25 million a year in January.
The Italian takes his side to Huddersfield Town on Saturday where Kepa Arrizabalaga, the world's most expensive keeper at 71 million pounds ($91.43 million), is set to make his debut.
The move was created to ease disruption in the first few weeks of the campaign after a number of high-profile players including Alexis Sanchez, Philippe Coutinho and Virgil van Dijk missed matches at the start of last season due to transfer speculation.