Rephrasing and Reproducing some part of the Introduction of a Research publication without citing that publication. Is it plagiarism?
Unequivocally, yes. Plagiarism isn't limited to results or figures alone. As for automatic plagiarism-checkers, they will make even less of a distinction, and will almost certainly flag it as plagiarised if a certain contiguous amount (I am told it is 2 sentences, but I don't have a confirmation on this).

You mentioned rephrasing - this could help avoid automated checks, but if the changes are trivial (changing voice, shifting subject/object, substituting synonyms), a reader may spot and report this.

That aside, why would you want to reproduce it? Different papers should have different introductions if their motivations are different. Is it to save writing time/effort or something else?