Januzaj came on with a little over 20 minutes remaining and showed why Belgium are keen to persuade the teenager to represent the country of his birth, rather than Albania.
"I felt great and shocked because I played for about 25 minutes and that was my first (league) game," Januzaj, whose parents are Kosovar-Albanians and who came through the Anderlecht academy before joining the youth ranks at United in 2011, told the club's website (www.manutd.com).
"I think I did some good things but it is only my debut so I have to keep doing well on the pitch. I appreciate the very good fans, they gave me a bit of power when I came on the pitch. I am very happy."
United manager David Moyes, never afraid to give youth its head as he proved at Everton, said Januzaj, 18, had impressed in the pre-season and was ahead of schedule.
"I would have introduced him earlier if the games had been suited but the start of the Premier League has been a bit more difficult," Moyes said. "I thought if I got the opportunity I'd introduce him and I considered playing him from the start.
"His performance when he came on shows that would have been merited. He did a lot of good things, he moved with the ball well. The boy's going to be a good player, he's 18 years old and he's a good talent."
Should he decide to play for Belgium, Januzaj will further bolster a squad brimming with top talent.
However, the player spotted by Moyes's predecessor Alex Ferguson, has never committed himself to Belgium and is being sought by the Albanian national side.