Celtic boss Neil Lennon says the demise of Craig Levein has left "a bitter taste" in his mouth.
The Gorgie club announced that Levein had been relieved of his duties as manager the day after Hearts' 1-0 defeat at St Johnstone on Wednesday night, which left them level with Ladbrokes Premiership bottom side St Mirren on points.
Disgruntled away fans had chanted against the former Hearts defender at McDiarmid Park while a 'Levein out' message was subsequently scrawled on the Foundation Plaza outside the main stand at Tynecastle.
When asked about Levein's departure, Lennon remembered the 'Hang Neil Lennon' graffiti sighted in Edinburgh a year ago following a derby match when he was manager of Hibernian.
"I am very sorry for Craig," said the Northern Irishman, who revealed skipper Scott Brown is struggling with a thigh strain ahead of the Betfred Cup semi-final against Hibs at Hampden Park on Saturday. "I know how much he put into the job.
"Obviously the results weren't what were probably expected for Hearts. He was a good football guy and gave everything for the club over the last 20 years.
"It leaves a bitter taste in the mouth the way some people have reacted or treated him in the past few weeks. The graffiti. They like a bit of graffiti over there, I know that from my own experience.
"For a guy who for the majority of his football career and life has been associated with Hearts, I think he deserves better. But he has kept his dignity and I wish him well for the future, whatever he decides to do."
The former Scotland boss will not return to his previous position as director of football at Tynecastle and has stepped down from the club's board, although he will stay on in a lesser role until the end of the season.
Assistant manager Austin MacPhee will take charge for Sunday's Betfred Cup semi-final against Rangers at Hampden Park.
"He will be sore," said the former Celtic midfielder. "I'm sure it will be bitterly disappointing for him, it meant the world to him I'm sure.
"That's the reality of the job I'm afraid, but there are ways you can treat people and the lack of respect at times for people in this position is appalling really.
"Not just the manager, sometimes the respect towards the players is appalling. If a player has a bad game, the vitriol they have to put up with is inexcusable."