Garry Monk says he felt betrayed and hurt by the actions of Pep Clotet when the pair were together at Birmingham City.
Monk was sacked as manager of Birmingham in the summer before taking over Sheffield Wednesday earlier this season and Clotet swiftly took over at St Andrew's.
Monk worked with Clotet at Swansea and Leeds before asking him to join the coaching staff at Birmingham.
Ahead of the pair's clash on Wednesday night at Hillsborough, Monk now believes he was wrong to be so trusting of someone he feels acted only to benefit his own career, and that he wants nothing to do with him.
'I don't speak with him,' Monk said. 'I think what’s most important to me when I’m assembling a staff around me is to give them opportunity. You show them complete trust and you hope they repay that trust with hard work and loyalty.
'Sadly not everyone has those values in their character. Some choose to pursue their own opportunities in the worst possible way.
'I think the most important thing for me is you live and you learn.'
Monks says he also regrets not listening to others who told him to be wary of Clotet.
He added: 'I didn’t listen to a lot of people in football circles who warned me about the type of character he is, but, yeah, that’s an error in judgment from myself.
'It was an error in judgment by me, but it’s an error I won’t make in the future.'
