I honestly dont see anything obvious here.
Bayern played 2 very bad games and Lahm just happened to be a RB in those. He also played RB on numerous other occasions and these games turned out to be fantastic.
He was simply one of the 11 bad players there on a team that played in generally bad. You could find 100 right or wrong possible reasons why it went down like that, with Lahm playing RB being just one of them. You could also wonder if the scoreline would have been any different if Lahm wouldnt have been RB those days. I mean there are also examples for amtches where he was in the midfeild and it didnt work out. You could consider all of these aspects.
But the evidence that you came up with in thsoe 2 games is simply not strong enough for what looks like a pretty superficial assumption.
I can find you other games (i.e. CSKA away, where a guy named Musa tormented him on the right flank, Lahm almost commited a penalty on him), but these two important games vs tough opponents are nice examples why Lahm isn't convincing on RB in current Bayern system. It's fairly obvious he's having trouble anticipating attacks and closing opponent wingers at 40-50m or even higher. For example, look at the beginning of Real's counter on 3rd goal and his terrible decision to follow up Isco or whoever that was.
The second part of this quote simply don't stand:
"It makes a clear difference wether Lahm is CDM or RB, Bayern is instantly a whole different and way more exciting team that way"
When you guys think about Lahm as a RB, you probably imagine him in his top physical shape and in a system that suits him, but that's not the case with Guardiola's Bayern. He's just not convincing in that high line and doesn't offer enough on offensive end to compensate that.
Donauwelle said:
You mean that ManCity match in which everybody fell to sleep and did not wake up anymore (last match of the group stage) after leading 5:1 on aggregate (3:1 first leg, 2:0 in this match). At the end of that match city won by 3:2 but did not win the group
City was only one goal away from topping the group. You know what would that mean in R16.