United threw away their early lead and will now have to overturn a deficit in the return leg when Bayern will be boosted by the likely return of midfielder Arjen Robben.
Van Bommel said: "I have seen United play well but I was disappointed with them in Munich.
"I think they thought they could hold on for a 1-0 win but, in the end, their negative approach backfired and ended in disaster for them. After the final whistle, I saw the look on the faces of the United players and they were in a state of shock and anger.
"It was like they couldn't believe what we had done to them and I'm sure that will have had an impact on them psychologically.
"United were conservative in the extreme and they may pay for that in the second leg now that they will be without Wayne Rooney.
"With Rooney out injured, we are now more confident than ever that we can win at Old Trafford and I'm convinced the return of Arjen Robben will make this all the more possible.
"We didn't have him available in the first game but on Wednesday his pace, skill and presence on the field will strike fear into their defenders."