United may be hit with injuries but they will be hoping to start the season with a win rather than immediately losing ground on their title rivals.
Mourinho sounded pessimistic that he would make any last-minute signings when talking to media on Thursday.
For the City players meanwhile, the focus is all about Sunday's trip to the Emirates Stadium as the season properly kicks into gear.
"I don't think just because they haven't signed a centre-back we should be sat here thinking 'we'll let Manchester United off the hook they shouldn't be challenging this year.' A club thats spent the money they've spent in the past few seasons and the players they've got. They trained on Monday, Tuesday, Wednesday and today, so you can imagine that they're not really fit and ready, but we have needs", said the boss of the quartet.
"I will have to focus on the players I have in relation to the first three matches, because after Spurs there is an worldwide break and I have a couple of weeks to recover people and prepare them better".
But this was only a "soft" closure - with some deals still able to be completed after this deadline.
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Whatever the explanation, the lack of new faces compounds issues surrounding the United squad heading into the Premier League season.
"Because it is very important how they feel and to understand how they think they can help or not help".
Richard Scudamore, Premier League executive chairman, said: "We know there are many passionate fans of Premier League clubs right across the Middle East and North Africa, and that BeIN Sports does an excellent job making the competition available to them". We go with everything we have in that first match.
The Portuguese believes runners-up are often portrayed as if they have been relegated and those who finish below them are presented as "serial winners".
City are the reigning champions after Pep Guardiola guided them to 100 points last season, but the Premier League years had previously been dominated by United, who won 13 titles under Sir Alex Ferguson.
"If we don't make our team better, it will be a hard season for us".