What the summer holds for Jose Mourinho and his Manchester United side nobody knows.Rumours are starting to circulate around Old Trafford regarding world-class targets and players set to leave the club.Among those linked with United are Atletico Madrid pair Antoine Griezmann and Jan Oblak, with the latter's agent reportedly flying into Manchester earlier this week to discuss a move.Oblak's potential arrival all depends on the future of David de Gea, who is once again on Real Madrid's radar and, should reports be taken seriously, wants to leave.What's worse is that Mourinho will not stand in his way of leaving if he so wishes, not to mention the fact De Gea recently put his £3.85 million mansion up for sale.Losing the 26-year-old would come as a huge blow to United and while there are plenty more fish in the sea, replacing him would be very difficult.Another Spaniard who could be on the move is Juan Mata, whose father admitted in November that he will evaluate his future at the end of the season.Mata has become a fan favourite since arriving at the Theatre of Dreams in 2015 but his strained relationship with Mourinho has been a problem in the past."This season we have entered a new path with Mourinho," Juan Mata Sr. told Turkish daily Fanatik. "However, at the end of the season we will sit down and evaluate our future more seriously."The Spain international's future will be far from his mind today, though, because today he turns 29. Many of his teammates have wished him a happy birthday on social media, but it's De Gea's tweet that's most intriguing.Not only did he aim a cheeky dig at Chelsea, but he provoked a huge response from United fans. Check it out.

Alonside a picture of Mata in blue paint, De Gea wrote: "Definitely red suits you better! Happy birthday my friend!"

De Gea clearly believes Mata looks better in United's colours than the blue of Chelsea, but fans had other ideas.

In the Twitter reaction below, United supporters all turned De Gea's tweet back on him by saying he looks better in red than the white of Real Madrid.

TWITTER REACTS