Grammar parsing for "if need be"

I have the following question. There is an idiom 'if need be'. The meaning is clear, but I can't comprehend it from a grammatical point of view. How should I parse it? 'if [smth] needs to be'? Why not 'needs' then? The question probably is silly...

Solution 1:

This idiom is a remnant from a time when ‘if’ constructions more rigorously required the subjunctive mood in the following verb.

To this day, the past subjunctive is used after ‘if’ when it has an irreal meaning (“if it were … but it's not”). It used to be that this was true also of the present tense, and as such, you would say, “if it be true” rather than “if it is true”.

In this idiom, ‘need(s)’ would in contemporary English most likely have a definite article attached to it: “if the need is [= exists]”; but the form without the article is, for whatever reason, the one that survived in the idiom.

So “if need(s) be” is no more than an older, fossilised form of “if the need is/exists” or “if the needs are/exist”, which is quite understandable even in contemporary English.

Solution 2:

Paraphrase : "If the need were to occur" : substantive + subjunctive.