I'm still not sure what our best eleven is but I think we're starting to see bits and pieces of what works. Assorted thoughts:
Back Four: Holgate or Kenny at RB. An absolute no to Martina unless [*shudders*] he's the only choice.
Back Five: Martina or Kenny at RWB. I'd like to see more of Holgate at RCB in this system.
With both Gana/Schneiderlin in midfield: Only one of Rooney/Sigurdsson/Klaassen should be on the pitch (unless it's a rare match where Rooney can play CF or drift in from the wing effectively).
With only one of Gana/Schneiderlin in midfield: We haven't seen it yet but I think you can get away with having two of Rooney/Sigurdsson/Klaassen playing centrally. Sigurdsson has played CM in a 4-4-2 for Iceland and Klaassen, by all accounts, played more of a box-to-box role for Ajax. With that said, you probably want Davies on the pitch.
Wide areas: Sigurdsson and Calvert-Lewin are not wingers and shouldn't play there, unless it's an emergency situation. You can get away with Rooney drifting in from the wing as long as he has adequate defensive protection behind him (i.e. not Martina). We look best with players who can actually run - like Sandro, Vlasic, Lookman, and Lennon - in the wide areas (who would've thought?).
Centre forward: At this point, Calvert-Lewin has earned the right to start. Rooney will be useful against teams that let us advance the ball into the final third easily and challenge us to break them down when we get there (basically just the home leg against Split so far). It'll be interesting to see if Niasse gets any more game time. Sandro can't play CF until he shows a willingness to actually run in behind the defense.
I really hope Koeman learns from what hasn't worked so far. Yes, Chelsea/Spurs/United are good teams capable of hammering anybody, but I think we could have at least made things difficult for them if we had provided any sort of threat, which you don't really get with slow players like Sigurdsson and Rooney drifting around in front of predictable midfield passers. We went out with no way of keeping the ball, which meant that it was always coming back at us, and you can only hold off a top opponent for so long like that. The City lineup only worked because Calvert-Lewin helped drag us out of danger every once in awhile with his runs into the channels.
Bournemouth are an athletic team and despite being a little over-aggressive at times, they have all the qualities to punish a slow, predictable opponent. I'll be disappointed if we get another Sigurdsson + Rooney + two DMs lineup for this one but, given the players rested mid-week, I fear that's exactly what we're going to see.