Di Maria arrived at Manchester with the pressure of being the Premier League's record signing, but showed no sign of early weakness with a string of impressive performances.
However, the Argentina international's season has since been punctuated by injuries, while a burglary at his house is also said to have had an impact on him.
A red card in last month's FA Cup defeat at home to Arsenal forced Di Maria out of Van Gaal's plans, but United have won all four of their games since to vault into firm top-three contention in the league.
Scholes wrote for the London Evening Standard: "I have been critical of my old club Manchester United under Louis Van Gaal this season and I have to say that I did not see this change in fortune coming.
"To the extent that when they go to Chelsea on Saturday, I think they have a good chance of winning the game.
"When I look back on the key changes, I still feel that Angel di Maria's red card against Arsenal was critical in allowing Van Gaal to build this new-look team.
"When I look at the side it seems to me to be a triumph of the underdog - Juan Mata, Marouane Fellaini, Ashley Young. All of whom looked like they could be out the picture at the start of the season and are now having a big impact."
