The Jambos experienced their equal heaviest defeat when losing at Tynecastle which has prompted a request for forgiveness from MacDonald who was on the end of all the carnage delivered by the rampant Hoops.
"That was as difficult a game as I've ever been involved in," he said.
"I'd just like to apologise to the fans for what happened. It was an embarrassment.
"We are disappointed for ourselves and for the fans.
"It was a bad day at the office for us and this is probably the most difficult time I've had in football.
"We didn't do ourselves any favours at the first couple of goals.
"Losing an early goal is the cardinal sin against Celtic. You try to get to 15 minutes and contain them.
"To be honest, you have to give Celtic a lot of credit. They were phenomenal."
He added: "The boys need to take it on board, learn from the mistakes we made and look at how Celtic played - and maybe take something from that.
"Hopefully we are much improved in the next game."
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