After much effort and an advertising break the owner of Virgin Radio managed to laugh off the remark as a joke, but there was no hiding the seriousness of his tone when Mr Macdonald - known as "Weel" to viewers of Evans' TFI Friday - made the accusation.
The ginger-haired presenter, who has made no secret of his fondness for the odd pint or six, was discussing what legalising cannabis would do to Britain - in his analysis it would grind to a halt because no one would be able to leave their chairs. His producer then said: "To my knowledge, you smoke nine or 10 joints a day, don't you?"
This Evans did not take well, replying: "Well I don't know about your knowledge Will, but to my knowledge not at all."
Unlike the usual banter that characterises Mr Evans' breakfast show, the exchanges between the DJ and his employee took an increasingly aggressive turn with Macdonald repeating the accusation: "Chris Evans smokes drugs, lots of them" and Evans becoming deadly serious: "No I don't. I'm going to sue you for that, I really am, I'm going to take you to court and I'm going to sue you for every penny and your flat, because I don't smoke lots and I don't have nine joints a day at all."
Listeners could only wonder where this exchange was going when it was interrupted by a traffic report. It would appear that some unheard agreement was then reached that would let the head of Britain's latest media empire get out of an increasingly fractious battle of wills.
Once they returned to the fray, Evans' underling ratcheted up the conversation to a ridiculous level, accusing his boss of having sex with rabbits. Evans was not placated and said he had things he could air about Macdonald which he wouldn't so he could not be counter-sued. Then Evans asked his friend the footballer Paul Gascoigne to call in and deny he took drugs and Evans' employee Danny Baker to say cannabis wasn't his drug of choice - cocaine was.
Macdonald met Evans on The Big Breakfast and was Evans' first employee and the man who devised Don't Forget Your Toothbrush. Presumably with a track record like that he can say almost anything he likes.