Opposite adjective for "professional"...

If he is starting his career, he is a professional (i.e., someone working in that business). The words we have for someone just starting are all rather pejorative in that they suggest that the person is unskilled. Perhaps it would be better to be a bit jocular and call him a 'rookie rental manager'.

Kedra opposite to professional would be unprofessional.
but If you use unprofessional to describe someone's behaviour at work, you are criticizing them for not behaving according to the standards that are expected of a person in their profession.

for example
What she did was very unprofessional. She left abruptly about 90 minutes into the show..., He was also fined $150 for un

Neither amateur nor unprofessional applies here, Butterfly wings. Do you understand that we are looking for a position name?

a position name?
the manager is being described is he not? Kedra says it ought to be an adjective?

Mister Micawber, thanks for your quick answer. That helps a lot !

Butterfly wings, yes, an adjective, you are right. Perhaps I did not make myself clear enough. I meant not unprofessional, but the point was to find the right adjective for those, who start working in the industry. Thanks anyway

Ofcourse you are welcome