The Portuguese star left Old Trafford last July for Turkish club Fenerbahce, and he revealed that Van Gaal's lack of confidence in him led to his decision to depart the Manchester club, per the Sunday Times' Duncan Castles:
He [Van Gaal] didn't use me a lot in pre-season. I was the player who had the least time in all the games and I was feeling, "This coach doesn't count on me."
He and his assistant were shouting at players like we were kids, like 18-year-olds. And I think, 'This is not the way to treat us because I am professional, I am training every day, I'm doing nothing wrong, I'm putting quality in the training.
Nani, 29, also added that he believes Dutchman Van Gaal's project at United is "not working."
The Portugal international arguably has a point given that United are still struggling to finish the current campaign in the Premier League's top four.
Under Sir Alex Ferguson, who took Nani to United in 2007, qualifying for the UEFA Champions League was a practical certainty as the Red Devils were perennial title challengers.
Van Gaal has been unable to return United to such a position following his 2014 appointment as David Moyes' successor.
As a result he has come under huge pressure in his job, and it has been widely rumoured that the 64-year-old will be replaced by Jose Mourinho in the summer, per Sky Sports' Matthew Treadwell.
For many, the Portuguese boss is the man to return United to the upper echelons of English football, and he would reportedly be able to persuade star goalkeeper David de Gea to stay at the club, per Bleacher Report's Dean Jones:
Nani was hardly used at all by Van Gaal—he has loaned out to Sporting in 2014-15—but he has shown decent form since his switch to Fenerbahce, netting seven times and providing four assists in the Turkish Super Lig this season, per WhoScored.com.
He clearly has little time for the former Barcelona and Bayern Munich boss' methods, and there are reportedly those who agree with Nani's assessment of Van Gaal still at Old Trafford, per the Manchester Evening News' Samuel Luckhurst:
Champions League qualification is still on the cards for United this season, but they have not been involved at all in the title race in the second half of the campaign.
It is indicative of United's reduced aspirations since Ferguson's retirement that a top-four finish would represent a good season for them.
As Nani suggested, such an attitude seems to indicate that Van Gaal is failing at United, and he may yet pay with his job.