"I don't think I've seen a player, probably since Ryan Giggs, who was as good as that," Butt said. "He was unbelievable. In my eyes he should have gone on to be a world superstar."
It's more than three-and-a-half years since Januzaj left United for Real Sociedad, and as he prepares to face his former club in the Europa League on Thursday, he is asked about Butt's claim.
"What he said, it's true," he says. "But the only problem is that I didn't have the right coach to get me on my level and to push me."
Januzaj's time at United encapsulates perfectly why the club have found it so difficult in the years since Sir Alex Ferguson retired in 2013. With new managers came new ideas, and while Januzaj flourished under David Moyes, he could not win over Louis van Gaal or Jose Mourinho.
He had only just turned 18 when he was named on the bench for the first time in Ferguson's final game, a 5-5 draw with West Bromwich Albion in May 2013. Handed his senior debut by Moyes the following season, he made 35 appearances in a breakthrough campaign. He scored twice on his first Premier League start in a 2-1 win over Sunderland in October 2013 and signed a five-year contract just two weeks later. His rise was so spectacular that he earned a place in the Belgium squad for the 2014 World Cup.
But the upheaval at United, of which Januzaj became a victim, was only just beginning.
Moyes was sacked in April 2014 and replaced with Van Gaal. Januzaj's performances the previous season had seen him inherit Ryan Giggs' No. 11 shirt, but with Van Gaal in charge he hardly got a chance to wear it.
A key part of Moyes' squad, Januzaj found himself on the fringes under Van Gaal. In the Dutchman's first year, he started just seven league games. The following season, in August 2015, Januzaj was handed a rare start in United's second game of the season and scored the only goal in a 1-0 win at Aston Villa. But by the end of the month he had been shipped off on loan to Borussia Dortmund. He would go on to make just three more appearances for the club.
Speaking about Januzaj a year ago, Butt ended by saying pointedly: "Talent gets you through the gate here but what will keep you here is character and commitment." Now 26, Januzaj has a different interpretation of why his time at Old Trafford was cut short.
"When Van Gaal came I was playing one game out of six, so it was difficult, and I was like, 'What am I doing here?'" he tells ESPN. "The first season under Moyes I was getting game time to show myself. I think I played 30 or 35 games. Then it was one game and five on the bench and with my quality, I didn't understand.
"The only thing that the coach can use against you is if you're not training well, but I know myself, I was training well. It's never been an excuse for me, I've always trained hard since I started football.
"People were always talking about me, I've done this, I've done that, and it was difficult. When you're 18 and you're being criticised like that, it's difficult. Especially when you're being criticised without getting games. I can understand it if I was playing 10 games and eight were bad games but it was not the case. Sometimes people outside don't understand these things.
"At Manchester United, the most disappointing thing was that I wasn't getting games. At that point I just wanted to leave the club. It was a difficult situation."
Van Gaal was eventually sacked in May 2016, but hopes of winning back his place under Jose Mourinho were ended when he was ushered out the door in a loan move to Sunderland to be reunited with Moyes.
Januzaj: Man United's youth players have it easier now
Adnan Januzaj explains why it was harder for him to break the Man United first team than youth players now.
"I wanted to leave United because I wanted to get games," he says. "I went to Sunderland, we all know it's not a team for me and not a team where I would enjoy my football, but I went there because I wanted to have games. I wanted to play, that was the only thing. It was last minute and it was like, 'You have to go.'"
Januzaj struggled, Sunderland were relegated and Moyes resigned. In July 2017, just three years after being tipped for the top at United, Januzaj joined Real Sociedad for €11 million.
"The club and the coaches have helped me to be happy here," he says. "Once you're happy in your life, you can be happy on the pitch. Here I have more opportunities than I had in Manchester and the more you play, the more you can show yourself and you can flourish.
"At United I didn't have that many chances, especially when Van Gaal and Mourinho came. When they came there it was more difficult for me because I was a young player. I was 18 or 19 and I just had to accept things. Before that I had people who believed in me like Ferguson. If he had stayed there longer I think I would have played longer for Manchester United."
Januzaj insists he will have nothing to prove when he lines up against United for Sociedad in their Europa League round-of-32 first leg on Thursday. He has ambitions to play in the Champions League and to be part of the Belgium squad for this summer's European Championship and the World Cup next year, but insists his focus for now is doing his best for the club who gave him a fresh start.
"I live very well here, it's a great city to live and the people are very nice," he says. "It's a big club here in Spain and everything is going very well for me so I'm really happy.
"We are an attacking team, we have good attacking players. It's always been a club that has had great players, we try to play attractive football and we have good young players so the club is going very well.
"I'm 26 and still young. Sometimes people think I'm 35 because I've been so long in the game. I have to focus here and the rest follows. If you perform for your club, the rest follows. I'm really happy here and enjoying myself and hopefully I can keep going like this."