Sheffield Wednesday boss Garry Monk says he made an "error of judgement" working with Pep Clotet.
The Spaniard was assistant to the Owls manager at Swansea, Leeds and Birmingham before replacing him at St Andrew's after his sacking this summer.
Clotet's Birmingham side visit Hillsborough on Wednesday evening.
"You live and learn. Unfortunately, I didn't listen to a lot of people in football circles who warned me about him," Monk told BBC Radio Sheffield.
"That's an error of judgement from myself. I don't speak with him.
"What's most important to me when I'm assembling a staff is to give them an opportunity and show them complete trust, and that they repay that trust.
"Sadly not everyone has that character. Some choose to pursue their own opportunities in the worst possible way."
Monk added: "I don't want to waste energy talking about it."
Following his sacking in June, Birmingham chief executive Xuandong Ren claimed there was a dispute relating to the use of Monk's agent in "all" of the Championship club's transfer deals.
Monk, who said at the time he would "not engage with those comments", spent 15 months in charge of the Blues, twice leading them to safety in the second tier.
"I'll be honest, it hurt me to leave that club after all that work that had been put in," he said.
"Then you receive that text, a text saying your job has been terminated, I was hugely disappointed after all that work."
Clotet, 42, would not discuss his relationship with Monk when asked about it in his pre-match news conference.
"Being very simple about the situation, it is a football game away from home against Sheffield Wednesday and the focus is on competing for the three points," he told BBC Radio WM.
"And we are very confident about our chances. This is the only thing that matters."
Birmingham are 15th in the Championship, two points behind Wednesday who are ninth.