I think the biggest mistake against Dortmund the first time around was to play 5 immobile passers in midfield (Wilshere and Rosicky on the wings, iirc). That was a bad bad decision by Wenger, though I suspect there weren't a whole lot of other options. My memory's a little fuzzy. This Dortmund team are built to kill on the counter, but so are we. No Theo will be a blow, but Cazorla being back is very important for us.
Ramsey and Arteta in midfield should hopefully evolve into Ramsey and Wilshere in midfield; I hope they can find a way to balance their game against each other's weaknesses, because in theory, that is a physical, dynamic midfield pair on par with Fabregas/Flamini. And those two didn't have an Ozil to play in front of them.
if we can set out at Old Trafford with:
Ramsey-Arteta
Theo-Ozil-Cazorla
Giroud
like... we could lose. But we really really shouldn't.