O'Neill chose to sell the centre half in January and may just be rueing his decision as Villa have stuttered in their charge for Europe.
The Claret and Blue only bought one player in the transfer window and that signing, Wayne Routledge, has so far been fruitless.
Many fans thought Villa's Northern Irish manager had a screw loose when he sold Cahill without replacing him and they have been proved right.
Curtis Davies was ruled out for the season, whilst Craig Gardner has suffered a string of injuries. That has left the once strong backline looking fragile and they have failed to keep a clean sheet for 19 games.
Cahill would have been perfect cover but he was never given a real chance to prove himself by O'Neill and he is now flourishing under Gary Megson.
He won't want to send Villa fans into the depths of despair but he would love the Trotters to come away with a victory on Saturday.
That victory seems a distinct possibility if you look at Villa's recent form. However, Bolton have been even worse and the Claret and Blue should expect to win this one.
They are likely to start with John Carew and Gabby Agbonlahor up front once again after Carew recovered from a knock picked up at Old Trafford last weekend.
Agbonlahor has been struggling for goals of late but he has managed to keep his place with Marlon Harewood still knocking at the door.
Bolton will again be without key players Kevin Nolan and Jussi Jaaskelainen as Nolan is suspended whilst the Finnish keeper is still injured.
Ricardo Gardner and Nicky Hunt are also missing through injury but Gavin McCann and Jloyd Samuel look set to make their first return to their former club.
It could look more like a game between Villa past and present on Saturday but let's hope that the present triumphs and the past doesn't come back to bite O'Neill.