All is not well at Manchester United.

Jose Mourinho has become increasingly frustrated with the players he has at his disposal in pre-season.

Man United have been missing a number of key players due to the World Cup, including Paul Pogba and Romelu Lukaku.

To make matters worse, Mourinho has failed to make any headway in the transfer market, with Fred, Diogo Dalot and Lee Grant Man United's only acquisitions thus far.

Their problems are so bad, that only five senior players trained on Monday ahead of their International Champions Cup clash with ​Real Madrid.

Mourinho has attempted to rectify the situation by calling on his players currently on holiday to return early and help out the team.

Marcus Rashford and Phil Jones have already cut their holidays short, and Mourinho wants others, including Pogba and Lukaku, to follow suit.

He said during Man United's tour of the United States: "I hope that the boys on deserved holidays take care of themselves a bit and that somebody wants to do what Rashford and Phil Jones decided to do, which is to be back a bit earlier to try to help the team because for the beginning of the season we are going to be in trouble."

However, these comments could actually land Mourinho in hot water.

That is because, according to the Times, Premier League stars that return early from their holidays are in fact breaching their own contracts amid concerns over player welfare and the potential for injury.

The text reads: "Clause 14 of the standard players’ contract drawn up by the PFA in consultation with the Premier League and EFL states that all players are entitled to a break of 'five weeks in aggregate over the year, of which three weeks may be consecutive'.

"The clause also states that 'clubs should not unreasonably withhold permission' for a player wanting a break, “subject to club commitments and international commitments”.

Ouch. Mourinho would perhaps not have been aware of that specific clause in players' contracts, but it could prove costly if action is taken.

Man United are not the only club where players have returned early from their holidays.

Trent Alexander-Arnold has come home early to join Liverpool's training camp in France, while Ruben Loftus-Cheek returned to Chelsea yesterday.

And that is concerning FIFPro, the world players’ union, who have expressed concern that players were being pressured into forgoing holiday.

A senior FIFPro source even told The Times that some players were being subjected to “emotional blackmail” by their clubs.

Whether action is taken on clubs that have had players return early from holidays remains to be seen.