I dont think O'Neill is gonna look for 'direct' cover for players like Carew as it will limit how flexible we are tactically. I may be wrong but I'd be very surprised if every time carew was injured he'd be replaced 'like for like'. I think any striker he signs will have different skills, giving us four different options, thereby ensuring the opposition wont just be able to predict the way we play all the time. For example it has been mentioned that signing someone like Bellamy or Defoe (just an example fella's!) would be too similar to Gabby, but to my mind thats a very shallow comparison in that pace is the only similarity. Either of those players would add something we dont have in the other three, Carews agression (granted without the physique) allied with directness.
And that theory also stretches to other signings. A question I've read repeatedly is if we get NRC and we also go after Sneijder, where will they all play? Again that is assuming we're gonna have a 'starting 11' that never changes unless forced to (like under DOL) again being too predictable. I see MON having a core 5 or 6 players that are regulars, with the other 5 positions being selected on merit and to suit the occasion. That also gives flexibility of formation and would explain why two of MON signings (Young and Maloney) are two of the most versatile players in the prem with the ability to cover any forward going position.
I may be massivley wide of the mark and Mr O'Neill may prove it by signing a Carew mk II but its just how I have being viewing both the tranfer rumours and, over a longer period, the positional experiments since Jan.