Eni said:
Can I substitute "necessarily" with "if necessary" in the following sentence?
He necessarily performs a set-up of a portal area.
Two points about the question rather than the answer
I think your question should read either:
Can I replace "necessarily" with "if necessary" ....
- or -
Can I substitute "if necessary" for ""necessarily" ...
I don't understand the sentence you are asking about - in bold above. I suspect that you actually mean "if necessary" and that one of those articles should be definite. Could you clarify what your sentence is intended to mean?