Di Maria failed to impress during his spell at United under Louis van Gaal last season and was sold to the Ligue 1 club last summer.
The Argentina international has been in impressive form for the French side this season, scoring nine goals and notching up 10 assists in the league for PSG.
And former Liverpool midfielder Redknapp admits that he is at a loss to explain why Di Maria failed to perform for United consistently during his spell at Old Trafford.
Writing in his column for the Daily Mail after Di Maria helped PSG dump Chelsea out of the Champions League on Wednesday night, Redknapp said: “I wonder what Manchester United fans would give to have Angel di Maria playing for them now in this form.
“I have no idea how he failed so badly at Old Trafford under Louis van Gaal when he’s capable of that pinpoint cross for Zlatan Ibrahimovic’s late winner. What a player!”
Di Maria has also scored three goals and made two assists in eight Champions League games for PSG this season.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news