Ferguson, 66, has built arguably the best squad of his 22-year tenure and former United, Celtic and Scotland player Macari sees no sign of his fellow Scot stepping down yet.
Macari told BBC Radio Five Live: "I think the big question is 'when is he going to retire?' but the way things are going at this moment, I think we are years away from that.
"Since he has been at Old Trafford his routine has been every morning from 7.30 until four in the afternoon. He is still doing that and I think that's the sign of somebody who wants to stay in the job.
"I just fear for the next manager who takes over at Old Trafford because he's on a loser!"