Pep Guardiola didn't have have any of Javi Martinez, Bastian Schweinsteiger or Thiago Alcantara at his disposal on Wednesday. He was always going to need to play Toni Kroos in a more defensive role than he's used to and play one midfielder that isn't actually a midfielder if he wanted to stick with a variation of 4-3-3 or 4-1-4-1, but his lineup wouldn't have been too weird if Philipp Lahm played midfield and Rafinha started at right back.
So everyone let out a collective gasp when Rafinha wasn't in the Bayern Munich team, wondering what they were going to see. What we got is not really a one-midfielder formation or anything that requires Thomas Müller or Mario Götze to act like midfielders. Instead, Lahm and Alaba are pinching into midfield, with Lahm looking more like a midfielder than the two. Alaba's actually getting forward somewhat occasionally.
Bayern's team is something along the lines of an unbalanced 2-3-5 where the only consistent fixtures are Mario Mandzukic up top, Kroos in the middle and the two central defenders. The fullbacks pinch into midfield and the other four attacking players just kind of run around. It hasn't confused Manchester United at all -- they're just fine with sitting in their two banks of four defenders and waiting for Bayern to get too fancy in the box and give the ball away.
False fullbacks (is that what we want to call them? I have no idea what we're calling this) to make up for Bayern's lack of available real midfielders wasn't the worst idea in the world, especially since United don't appear to know how to take advantage of the available space on the counter. But Bayern don't look like they can break United down, and David Moyes is probably smart enough to make some sort of adjustment in the second half.