Rather I doubt very much you have ever played football or understand it. That is all too clear from your post. Because if you did:
1. In football primary roles on a pitch never ever change unless express instruction from the bench have been given. In short you are never EVER supposed to do another player's job for him. i.e a defensive midfielder marking a striker, a striker tracking a defensive midfielder, a center back marking an attacking midfielder in midfield e.t.c...It's a basic law.
Occasionally you help out. That's it. Nothing more nothing less and only when they are in deep trouble. The Arsenal case doesn't not constitute deep trouble, because we were playing a high line in order or the midfield to push into the opponents half. It's thus up to the defence to do it's job. Thus If Vidic wasn't God knows were, Adebayor would have been easily stopped or would have been offside.
Thus for a defender or anyone else to blame a defensive midfielder for not tracking a striker. Simply because his marker was sleeping and out of position, in a high defensive line is a serious lack of realism when it comes to football.
Furthermore to still lay the blame at Hargreaves' feet, for a goal casued by Wes Brown standing and watching Fabregas get to a ball he can clear is even more stupifying.
2. When you are in midfield, as two decent defensive players, facing a player of Kaka's caliber, with a weak defence behind you, you must double team at all times. Failure to do so is criminal. Because that would mean he gets to run at your weak defence, who are at the time ill equipped and can't be trusted to deal with such a threat, with out significant help. Which 9 out of 10 times will result in out right panic for them and unforced school boy errors. This Carrick did though out vs Milan. All the goals we conceded bore that resemblance. Kaka or Seedorf running at our defence, then panic and errors
3. You can't be arsed to argue Kaka was a second striker simply because it's utter bullshit. Especially, when Milan clearly play a wingless, one striker 4-3-2-1 formation. With 3 deep defensive midfielders, lined up as two hard men on either side of a deep lying playmaker, With 2 attacking midfielders directly infront of them, hovering around a lone striker, with fullbacks providing the width.
The chief has spoken