Reo-Coker dropped after O'Neill row
O'Neill rejected newspaper reports on Friday that the two had to be separated by other players after grappling each other to the ground but said there had been a "verbal confrontation".
"There was a mishap on the training ground," O'Neill told the Villa website.
"It's a common occurrence between players but on this occasion, unfortunately, it was between the manager and a player.
"There was a bit of a contretemps and that is unfortunate. Nigel didn't train this morning and obviously won't play tomorrow," he added.
"I have dealt with the issue how I see fit and that's the end of it. Come Monday, we're back to normal again. I am the custodian of this place, my rules apply."