Di Maria, 27, is on the brink of joining PSG after he was pictured in Doha for a reported medical in the Qatari capital on Monday ahead of a £44.5 million move.
The Argentina international's imminent switch to the French champions comes just 12 months on from his record £59.7 million move to Manchester, where he failed to settle in Louis van Gaal's side despite enjoying a bright start to life at Old Trafford.
And Ferdinand, who won six Premier League titles with United, said Di Maria's reputation has been tarnished forever.
"He will now always have on his CV that he was a failure at Man United," Ferdinand told ESPN FC at a BT Sport media event.
"As a professional, I wouldn't want something like that on my record. If I could rectify it by staying and trying to prove myself, I would have done that.
"As a fan, I would have liked for him to stay there and prove he can perform for Man United. That is what we all wanted to see when the club paid so much for him last summer, but that isn't happening, it seems.
"I can understand why some fans are disappointed that he is leaving so soon, but this is a business.
"United may have looked at it this summer and Van Gaal has probably come to the conclusion that he doesn't fit into his system for whatever reason. At that point, you have to cash in."
Di Maria made just 32 appearances in all competitions for United last season, scoring four goals and providing 12 assists.