If by "loses" you mean "loses the general election", that could be a bit problematic.

In 2000, Bush mainly kept his mouth shut. But Gore's campaign manager made a series of public statements along the lines of "Gore has clearly won, and if this goes to Bush it's a travesty of the election." That didn't help heal or unite the country after the election; arguably, the division has remained ever since.

Trump could do the same thing, but do it much more loudly, bombastically, and in a more attention-grabbing way. The only prevention would be for him to lose so strongly that nobody listens, and everyone writes it off as the whining of a bitter self-deluded loser.

I mean him exhorting his followers to 'do something about it'.

Yeah, that could get... interesting. And deadly.