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On one hand, possessive case is one of the simplest topics and still there are few nuisances. Should I say:
Tom, professor assistant, was entering the lab.
or
Tom, professor's assistant, was entering the lab.
Tom doesn't belong to professor, but words professor and assistant definitely have the possessive link - assistant OF the professor
Certainly, Tom does not belong to professor but the professor can still say, "He is my assistant", can't he?
That said, "Whose assistant? ~ Professor's". The possessive is fine there.
Out of those two,
Tom, (the) professor's assistant, was entering the lab.
is okay.
Tom, the professor's TA, entered the room. is how it would be said in American English. Substitute "assistant" for "TA" (teaching assistant).
