Villarreal, Celtic, Aalborg
The Champions League group stage draw is fairly good: United avoided big guns like Bayern, Juventus or Atletico but were not spared the Battle of Britain hype.
Villarreal were the first to come out: we met them during the ill-fated 2005/06 campaign where they eventually marched to the semis. Two 0-0 draws are unlikely to be played this time.
Celtic, of course, we faced many times, most recently two seasons ago: a 3-2 win at Old Trafford and a dismal 1-0 defeat away wasn't perfect but we won the group.
I'm fairly certain that we've never met Aalborg yet - only other Danish teams. FC Brondby, of course, conceded eleven over two games in '98 while Kobenhavn finished third in the group in 06/07, losing 3-0 at Old Trafford but beating us with an Allback goal in Copenhagen.