The Red Devils overcame Sociedad in the last-32 stage with a 4-0 aggregate win.
Speaking after a goalless draw with the second leg on Thursday, Januzaj said: "Compared to the first game, today we showed we are much better team than the first game. We conceded four goals too easily but now we have to focus on the league.
"When the penalty did not go in, we thought it is not for us anymore. We had another chance just after, that is two, so if we scored it would be a bit more exciting.
"We were much more dominant in the game today, we closed the gaps and it was harder for them to score. We had good opportunities to score too and lost, that is it.
"I think Manchester United are one of the favourites, with the team and players they have got, they have a lot of chance to win this.
"The public (supporters) is always important for the team, they are there to push you and we we had them in the first leg it would have been a different game."