The limits of self-plagiarism
You have a misconception. Plagiarism isn't about the actual words. It is about ideas. Copyright can cover the expression of the ideas (the words). In other words, you can plagiarize even with a complete rephrasing of the original. But it is easy to avoid.
To avoid self plagiarism all you need to do is cite the original work. I'm assuming that the words you use are expressing the same idea though in a different context. And if you cite and you hold copyright then there are no issues at all. If you don't hold copyright then you either need to get permission from the holder or depend on one of the exceptions to copyright law (which varies from place to place). You may well be able to claim fair use. You might also be able to claim that your reuse of the sentence doesn't reduce the value of the original. Those are valid in some places. And some ideas can only be effectively expressed in one way.
The reason to avoid self plagiarism is a bit different from that to avoid ordinary plagiarism. A reader of a new work may need to find the complete context of the ideas. This includes other words and references, etc. If you don't cite, then that chain is broken for a researcher. This also applies to ordinary plagiarism, but the need to not claim the work of others as your own is typically thought to be the more important part there.