The Premier League champions take on Bundesliga winners and UEFA Champions League holders Bayern in the first leg of their European quarter-final at Old Trafford on Tuesday, with the Germans the clear favourites to progress.
Pep Guardiola's side have only lost once in all competitions this season, to Manchester City in December, and they arrive in England a week after securing their 24th domestic title.
"I'd say, going into this game, we're hopeful rather than confident," the former right-back said in United's matchday programme.
"That's just being realistic - Bayern are at the absolute top of their game.
"You're talking about a team at the absolute peak of its powers, while Manchester United is in transition.
"There's no avoiding the fact that this is the toughest draw United could have faced."
