The Australian bagged 51 goals in just 88 appearances for the Old Firm side during a three-year spell at Parkhead between 2007 and 2010.
McDonald went on to spend three seasons with Middlesbrough before switching to The Den last summer.
And although he took some time to find his feet in south east London, a goal and an assist this afternoon had manager Ollie purring afterwards.
“What can I say?” said the Millwall manager.
“Absolutely brilliant I thought, everything he did.
“He looked sharp, he looked bright, he looked as good as he was when he was playing for Celtic.
“That was a hard game, well done to him and I’m delighted.”
Holloway added: “From this time last year to now – what about that?
“And he looks as good as he did when he played for Celtic.
“He deserves the credit.
“What has changed is everything about him and I’m so proud of him.
“He was chasing people everywhere there, he don’t normally do that.
“Look at him - he is a centre forward and he scored.
“That’s what it is all about and Millwall, we deserve that don’t we?
“Working class people pay a lot of money to come here, we deserve a good shift and even though we almost let that (game) go I think our fans will go home and say that wasn’t a bad shift.”
Another Lion singled out for praise was 20-year-old Jack Powell, who kept his place in the side after making his first start in the midweek 2-0 cup defeat against Southampton and provided the free-kick for McDonald’s first half opener.
“Outstanding,” said Holloway.
“I thought he played really well against Southampton the other day.
“Personally I felt I was brave to do that. Maybe I was silly because I should have changed it a little bit before and rested a few particularly attacking players.
“But I thought he shone the other evening.”
Ollie added: “Unfortunately Jimmy Abdou has lost his grandfather this week and I had to let Jimmy go to his funeral today.
“I believe that is the right thing to do to pay respects to obviously somebody that was a wonderful man if he is anything to do with Jimmy Abdou, so that left an opportunity.
“I could have done other things but I wanted to just reward the boy for his efforts because I know he is a nice passer but you never know how they are going to respond in a big game at The Den.
“To be fair he looked like he has been there all his life.”
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