Pym was left out of the squad for today’s 3-0 Championship win over Birmingham City at the Weston Homes Stadium.
Social media was awash with rumours of a falling out between player and manager overnight, although club officials initially hinted Pym was ill.
Ferguson set the record straight after the game.
“As a manager these things sometimes happen. It’s not ideal – particularly the timing of it,” Ferguson said.
“I like to keep things in the dressing room. I knew the question would come and I’ve had to make a decision.
“It’s going to be difficult for that boy to get back as he overstepped the mark on Tuesday night.
“Christy has apologised for it, but I’ve got to have discipline and if there is criticism you have to take it like a man.
“We’ve got a really good squad and I’m not isolating Christy from it at all.
“But there are certain things as a manager you have to do and abide by, but this was not one of those.
“There are certain beliefs I’ve got that things like that shouldn’t go on.”
“But it’s gone now. I’ve made a decision and I’m sticking by it.
“It’s been a tough decision whether to bring Dai in or keep Christy in the team.
“Dai was the man of the match in the game he played in the Carabao Cup even though we were beaten.
“He made some very good saves and made another couple today while also bringing a real calmness to us.
“I’m delighted with his performance and the clean sheet.”
It was a first Posh start in the Championship for Cornell who moved to Posh from Ipswich in the summer.