Nowt wrong with Wheater - much rather him come into the team than the likes of Andy O'Brien & Danny Shittu as was the case until last season. As for Zat Knight, I think he's been a better player since Coyle came in - & much, much improved on the player we saw under Megson when his confidence must have been rock bottom.
Should Cahill go, Cuellar from Villa doesn't fit Coyles plans - too old & picks up injuries. As said already, I Onuoha could be a perfect fit at the right price.