Without defining "professionalism", this is a useless discussion. Professionalism, to me, implies a certain standard of conduct: Delivering what you promise, when you promise it. Communicating clearly. Refusing unethical requests. Not flaking out at a critical moment. Keeping your cool even when you're a bit frustrated. Refraining from conduct that will discredit your employer, clients or partners. Being clear about…
The opposite of amateur is expert.
An amateur can be very expert. An amateur is someone who does something because he wants to, for love of the work or subject. It has nothing to do with expertise or competence at the job.