Reds coach Aurelio Vidmar said his side would now have to be extra cautious going into a foreign competition.
"A lot of people have already started thinking a little bit further than that first game, but they're going to be tough," Vidmar told AAP.
"They've had a fair bit of experience playing in that competition last year, this is the first time we're actually going into the tournament.
"Every team in our situation dreams of playing Manchester United, but our sole focus now is on playing Waitakere."
Good news for Adelaide is that inspirational goalkeeper Eugene Galekovic and defender Scott Jamieson will be available for the play-off match.