MacLeod has been stunned by the collapse of the Dons who lost three matches in succession and then struggled to hold on for a draw against Motherwell last weekend after an exhilarating eight-game winning run.
And he insists they have to get something at Celtic Park or their challenge will be over.
He said: “Last year Aberdeen put in a fantastic challenge and with about six or seven games to go they were still there. But now you’re looking if Celtic win at the weekend then it’s just the end of October and it’s finished.
“It looked as though there was going to be a challenge to Celtic from Aberdeen as they looked very comfortable, they were in control, they’d beaten Celtic. So you thought there was a challenge but now you feel there’s no challenge.
“When they beat Celtic during the run they were on you’re thinking they’ve got a chance here. I’d have expected them to go the other way and that no-one would get near them. Now all of a sudden everyone has taken points off them and it’s not just an unlucky 1-0 they’re losing. St Johnstone scored five against them at Pittodrie.”
MacLeod, the midfield powerhouse of the 1980s, is perplexed by the collapse but hinted that it may be a mentality issue.
He said: “You look at the Old Firm and when they’re at the top of the table and things are not going well they can still grind out results, whereas Aberdeen it’s just gone flat for them.
“What they need is a good result, a good performance and that can maybe get them back on track because you don’t become a poor side overnight. That’s why it’s so important for Aberdeen to get something at the weekend. There is something not right there just now but hopefully it’ll change.”