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After being unveiled as a PSG player, Messi met many of his teammates and took part in his first training session on Thursday.
He linked up with Mbappe, who welcomed him to the club with a post on Instagram and shared pictures of the pair embracing.
Messi has already expressed excitement over the opportunity to play in a devastating front three alongside Mbappe and his close friend Neymar.
But according to Gazzetta Dello Sport, Mbappe has a very different feeling and is "bothered" about the sensational capture of Messi, who has signed a two-year contract with the option of a third.
A world star in his own right, it's said the 22-year-old is worried he might end up being in the shadow of Messi and Neymar, who were part of that glorious Barcelona trident alongside Luis Suarez.
The World Cup winner joined PSG from Monaco in a £163 million move and has developed into one of the most exciting attackers around, scoring an incredible 132 goals in 172 appearances for the club.
His future in the French capital is very much up in the air, however. Mbappe has a year remaining on his PSG deal and continues to be linked with a switch to Real Madrid.
In fact, AS say PSG expect to lose Mbappe for free next summer and, in a quite sensational plan, want to bring in Cristiano Ronaldo as his replacement.
It's claimed Mbappe and Real have said "I do" to one another in private.
During the press conference for Messi's unveiling, PSG president Nasser Al-Khelaifi was asked about Mbappe's situation and was adamant he would be staying put.
"Everyone knows the future of Mbappe. He is a PSG player," he said.
"Kylian is very competitive, he wants to win, he has publicly said that he does not want to leave the team and there are no doubts or excuses of any other kind. Mbappe stays'.