Margaret, from Rochdale, rang the club's television station MUTV during a phone-in to complain about the 'winger-less' formation that saw them fall 2-0 down against Braga in the Champions League in midweek.
They came back to beat the Portuguese side 3-2 - but Margaret was not entertained or amused.
"Please don't play the diamond at Chelsea. If you want me to have a word with the manager, I will," she told pundits and former United stars David May and Quinton Fortune.
"I've seen every good player you've ever had. And they weren't that stupid, and giving goals away, at the beginning.
"Have you not noticed all the goals you're giving away when you're playing that stupid diamond position? We have to go back to playing normally and catch up.
"It isn't entertaining when you're 80 years old to watch goals going in from the other side.
"I don't like it. It's going to give me a heart attack and I'm 80. You wouldn't like to give me a heart attack.
"I have an opinion and it's certainly not the diamond. They don't know what to do — they run around like lost sheep."
Ferguson, famed for his half-time 'hairdryer' - which would almost certainly have come out on Tuesday night - saw the funny side.
"I'd better not upset her. Did you hear her on MUTV?" he said. "She's 80, phoned up and said 'I'm fed up watching this diamond'.
"She said 'Tell that manager to stop playing that diamond'. So I better not play it."
And it's a good job he did - because Margaret had a stark word of warning for the Scot, 70, should he stick with the diamond.
"I've supported United longer than he has!"