United had long been touted as the favourites to secure the 19-year-old's services, however the Brazil international agreed to join the big spending French side on a four-year-deal after PSG agreed a fee reported to be in the region of €45 million (£35m) with the South American heavyweights.
PSG have already made a number of expensive signings this summer, with Thiago Silva, Zlatan Ibrahimovic and Ezequiel Lavezzi all agreeing to join Carlo Ancelotti's side as they look to secure their first Ligue 1 title since 1994 and mount a challenge for success in this season's Champions League.
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Ferguson believes that PSG's spending is detrimental to the game, however, he accepts that the French giants must now be taken seriously as a force in European football.
He told the club's official website: "I find it quite amazing that a club can pay €45m for a 19-year-old boy.
"To make a mark on everyone - to tell everyone that PSG are here - they've signed Thiago Silva from Milan and Zlatan Ibrahimovic.
"They must have spent about £150m in the last month. As you know, the only deterrent to that is Uefa.
"In the conditions of European football, you don't qualify for Europe by winning the league or coming second - you only get in by invitation.
"That's where, hopefully, Uefa can have some power. When somebody's paying £45m for a 19-year-old boy you have to say the game's gone mad."