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The Belgian burst onto the scene under David Moyes in 2013/14, scoring twice on his first league start against Sunderland.
He was the bright spark in a dire season for United but then found himself out of favour under both Van Gaal and Jose Mourinho, being loaned out to Borussia Dortmund and Sunderland before eventually leaving for Real Sociedad in a permanent move in 2017.
In a new interview ahead of what could be his first meeting against United since leaving, he singled out Van Gaal for not supporting him enough during their time together.
"When I played my first season at United and came through the academy, people were showing me love," Januzaj told the Daily Mail.
"The coaches were letting me enjoy my football.
"Then obviously when a coach (Van Gaal) comes who doesn't show you love, it gets more difficult to prove yourself and be happy.
"Even if you had a bad pass, Van Gaal would shout at you. For young players that isn't good, you have to teach them. A coach is like a teacher, that's what I missed the most.
"As young players we just had to sit there and wait for our opportunity to play for 10 or 15 minutes. Okay, we can show a glimpse of our quality but you cannot show how good you are."
Van Gaal, however, was not having it and refuted the claims from Januzaj.
"I have absolutely no need to defend myself against so much injustice!" The Dutchman wrote in an email sent to The Athletic.
"The self-image of players leaves the wishes! And my career as a coach/manager shows just how many young players could take their chances under my leadership!
'With a kind box, LvG," he finished.
Januzaj played 28 times in two seasons when Van Gaal was at the helm and had to make do with playing as a wing-back.
Still in touch with Sir Alex Ferguson, Paul Pogba and Luke Shaw, the Sociedad winger did say he would have fared better if he had a manager like Ole Gunnar Solskjaer working with him.
"You can see Ole is really nice and loves the players from the academy. For young players like Marcus they are very lucky to have a coach like that.
'When you are a young boy and you're being criticised for things you never did - the coach's excuse was that I wasn't training very well - I couldn't understand. You play like you train. If I was doing well in games, I don't think I was that bad in training.
"People had a bad image of me and that's the one regret, that I had lot people questioning my mentality."
Januzaj, now 26, has racked up 109 appearances for Sociedad since making the £9.8 million switch to the Basque outfit.
Sociedad play United in the Europa League round of 32 first leg at a neutral ground in Turin on Thursday, with the reverse fixture taking place at Old Trafford next week.
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