OK, now the game is over I hope people can read with an open mind and not permanently thinking there's an agenda in everything.
I couldn't go into it earlier because I didn't want to lay it all out (and it was all apparently complicated enough already when you have to explain fullbacks can attack AND defend).
Müller or Robben and López or Hagi were very interdependent decisions. I went with Robben assuming Gio would go López. I knew Hagi made for a sexier teamsheet but my worst case scenario was playing against López with Müller and my best case scenario was playing against Hagi with Müller. Robben was tactically a good halfway house.
Where's the link?
As you all heard by now, Koeman wasn't the quickest, which obviously is a negative. In this game, the relevant implication was that even with Thuram partnering him I couldn't live with both López and Vieri's pace. If Thuram tries to pick up López, that leaves Vieri to skin Koeman, which effectively prevented me from having a high defensive line.
The deeper my defensive line the less likely it was Gio could transition effectively and the less pace in space could be exploited. My benefit from having Koeman though was I could execute such transitions, which in turn meant Robben alongside Cavani upfront could keep Gio's defensive line pegged back as well.
The more I stretched the play and increase the distance between the lines the better placed I was to control the game as a result of having better passers and more players who could get involved both in defensive and offensive phases (I hope people actually realise this is bloody obvious when you compare what Brehme-Sagnol offer vs. what Bergomi-Carboni do).
THAT is why Robben was important. If I couldn't play a high line due to Koeman's pace I may as well use his exceptional attributes to still gain an advantage by forcing Gio to also defend deep. He never made any explicit remarks on that, no one asked or cared, which indicates I overthink this (even if it is evidently crucial).
Now enter Hagi... Pace is no longer such a high priority concern, particularly when Hagi would likely come deeper and get invoved in buildups rather than be up there trying to beat an offside trap.
It means I can defend high up, which means there's now far less distance between the lines. Koeman and Popescu are suddenly quite handy as fallback options during attacks, Cocu and particularly Schweinsteiger are more involved around the edge of the box, Brehme and Sagnol are taking far less risks purely by virtue of cutting the distance to recover in half.
At this point though it all gets rather congested. I'm applying more sustained pressure on Gio but everything is more compact. The implication is this is no longer the right game for someone who exploits space (Robben) but someone who creates it where there wasn't any (Müller). That's when Müller would come on and prove a game changer.
Hagi, great as he was, effectively would have been Gio shooting his own foot. Probably best I had no time to lay this all out, no one would listen anyway assuming it's all too convenient.
It was actually the move I was hoping he made from the off.