There is an article which contains a section describing preliminary materials, that is, results which have already been published elsewhere and on which the current work is based.
This section is not supposed to contain any new result.
It appears that part of this section (one to two pages) is, up to minor modifications, a copy-paste from another published article.
The other article is from a collaboration of the author of the present paper with other authors. It is cited somewhere in the current article but not as the source of the copied part.
The context and methods of the articles are very similar (hence the need for similar tools), but beside the copied part, the contents and results are different.
In particular, there does not seem to be any plagiarism in the actual novel part of the article in question.
Is this a case of acceptable self-plagiarism?
There are a lot of questions in academia.SE about self-plagiarism but I couldn't make my mind about this case from what I've read.
EDIT: Thank you for all this helpful contributions. As referee for this article, I've chosen the option to ask directly the editor's opinion.