The side have struggled in the wake of Fergie's retirement and David Moyes' appointment as boss, and there are plenty of fans who would be more than happy to see Sir Alex back on the touchline rather than watching on stony-faced from the directors' box.
As the bookies say on their own blog, via Aidan Elder:
Yes, it appears it was made by someone who has no idea what Sir Alex Ferguson looks like, but still – it’s Fergie back at Old Trafford – what more do United fans want?
It even says ‘in case of emergency, break class’ on the front! Like it would on a bus or some other enclosed space if was an emergency! Because David Moyes’ management is so bad, it’s an emergency for United. Ha ha! We’re so edgy.
Is this enough of an emergency? According to Sir Alex back in October, as he was promoting his latest autobiography on U.S. network PBS, there's no circumstances under which he'd return to the dugout, via BBC Sport:
I'm not interested in managing again, getting myself worked up. United are in good hands. David will be fine. He's a good manager.
[...] I made my decision. The timing was perfect. There is no way back for me now. I've got a new life.
That may well be true, but it's not going to stop some folks pining for it and giving bookmakers the chance to do some cheeky guerilla marketing.