6 Anthony Martial found himself in the firing line after leaving the club’s US tour to attend the birth of his second child.Mourinho made little secret of his displeasure at the Frenchman’s failure to return to the fold as quickly as he had expected, but 22-year-old Martial insisted his family would always come first.The problem with Pogba 6 6 Mourinho hoped to strengthen his squad significantly during the summer transfer window, but it ended without the arrival of the commanding defender he desperately wanted.Despite being linked with the likes of Raphael Varane, Harry Maguire, Jerome Boateng, Diego Godin and Yerry Mina, United added only Brazilian midfielder Fred, young full-back Diogo Dalot and veteran goalkeeper Lee Grant to the mix.Speculation persists that relations between Mourinho and the club’s executive vice-chairman Ed Woodward are strained.World Cup wait If things have been tetchy off the field, they have been little more relaxed on it.United stuttered through pre-season as they lost to Liverpool, Real Madrid and Bayern Munich, and were little more than functional in a 2-1 victory over Leicester in their Premier League opener.However, they turned in a dreadful display to go down 3-2 at Brighton on Sunday, with the scoreline flattering them somewhat.Brighton were fully deserved of their victory and Lukaku and Pogba’s strikes against the Seagulls were more like consolations than actually getting United back into the game.They were completely disorganised throughout and did not merit anything from their dreadful performance. 6 Martial’s future at Old Trafford is uncertain Paul Pogba is reportedly not happy under Mourinho and wants to leave Man United United failed to land their top transfer targets this summer The manager is under immense pressure at the Old Trafford helm after a difficult few weeks took a turn for the worse on Sunday when the Red Devils were beaten 3-2 at Brighton in the Premier League.Mourinho has cut a frustrated figure as his team have laboured through pre-season and the opening weeks of the new campaign, having also struggled throughout the transfer window.There are also reports of unrest behind the scenes with the boss and the United board, with claims they are no longer convinced of The Special One’s ability to lead the team forward.So will Mourinho’s third season curse come back to haunt him at Old Trafford?Here are five key reasons behind United’s summer of discontent…Window woes The manager was less than impressed after having to start pre-season without some of his biggest names after France, England and Belgium enjoyed extended stays at the World Cup finals and severely depleted his resources.Paul Pogba, Romelu Lukaku, Marouane Fellaini, Ashley Young, Jesse Lingard, Phil Jones and Marcus Rashford were all missing, although most answered Mourinho’s call to cut short their post-tournament breaks.Martial War? Paul Pogba won the World Cup with France, but still captained Man United for their first two Premier League fixtures Is the end near for Jose Mourinho at Manchester United? 6 Paul Pogba has been something of an enigma since his club record £89million switch from Juventus in August 2016.His form on the pitch has been patchy to say the least, and rumours of a fractious relationship with his manager have refused to go away.Pogba did little to stem the speculation when he replied, “There are things that I cannot say otherwise I will get fined” when asked if he was happy at Old Trafford, but Mourinho, who has made the Frenchman his captain, has repeatedly insisted there is no issue. Pitch battle Man United suffered a shock defeat at Brighton on Sunday 6 Will this be Mourinho’s last season in charge of Man United?