Januzaj, 26, came through the academy at Old Trafford but left for Spain in 2017. United and Socieded have been drawn together in the round of 32 of the Europa League with the first leg -- being staged in Turin because of coronavirus restrictions in Spain -- set to be held on Feb. 18 but Januzaj insists he will feel no added motivation.
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"I don't have to prove to anyone, I know my qualities," Januzaj told ESPN in an exclusive interview. "I don't have to prove to anyone how good I am, I just have to believe in myself, get my games and be happy.
"I'm really happy to go back there, the club I love. Once I'm on the pitch, there's going to be no friends because obviously I'm playing for another club now and also I want to win the game. I want to go through with my team. I'm really happy to go back there to see the people but once I'm on the pitch there will be no friends."
Januzaj's history at United is complicated. He was on the bench for the first time for Sir Alex Ferguson's final game in 2013 before becoming a teenage star under David Moyes the following season.
His performances earned him a new five-year contract in October 2013 and a place in the Belgium squad for the 2014 World Cup.
But when United replaced Moyes with Louis van Gaal that summer, things began to go downhill.
"I had Fergie and David Moyes, coaches that were believing in me, but then some others came in and they wanted a bit more experience in the team," Januzaj added.
"I was a young boy and when you're a young boy you have to shut up and do what people tell you to do.
"I didn't have the right coach to push me. For example 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?' The first season under Moyes I was getting game time to show myself. At Manchester United, the most disappointing thing was that I wasn't getting games."
After being a bit-part player in Van Gaal's first season in 2014-15, Januzaj was loaned to Borussia Dortmund in August 2015.
Jose Mourinho replaced Van Gaal in July 2016 and Januzaj was almost immediately loaned to Sunderland, before cutting his ties with United permanently with a move to Sociedad in 2017.
Januzaj appeared destined for a glittering career at Old Trafford when he was handed a new long-term deal as an 18-year-old in 2013 but, looking back, he said he would never have signed had he known Van Gaal would be in charge less than a year later.
"If at that time I could have chosen to leave the club the year before he came then I would have," he said.
"But at that time United wanted me to sign a new contract and I thought that I could be there for many years. If I knew that these coaches were not going to believe in me I would not have signed, I would have gone somewhere else and enjoyed my football.
"At that point I just wanted to leave the club. I didn't want to go on loan, I wanted to get sold so it was a really difficult situation. All of it is experiences and I know I won't make that mistake again so I can just enjoy myself and my football."