I understand it is easier to cope with a defeat by playing down United's chances in advance, but I expect it to be a tight game. Huge score-lines almost never happen when you expect them, least at such a stage. Bayern München will play with a lot more caution than in the Bundesliga, they always do in the Champions League.
One or two moments of brilliance at the right time from Rooney & Co. and it could become a very thrilling encounter.
On a side-note I would worry a lot more about the 1st than the 2nd leg, considering Bayern's incredible form away from home and their past struggles at the Allianz Arena in 2nd legs.