I'm a PhD student currently applying for postdocs and while I have secured my first and second letters of recommendation from faculty, the professor that had agreed to write me my third has unfortunately passed away recently.
I don't have a fourth professor sufficiently familiar with my research to write a strong recommendation letter. If I were to ask one, it would be one of those "good but also kinda filler" letters written by someone who knows me but doesn't know me.
Alternatively, I could ask a postdoc I worked closely on several projects for a letter. He would be able to write a strong letter, but of course that's still not ideal because he's a "mere" postdoc.
Which one would you recommend? A good letter written by a professor mostly as a favor, or a strong one written by a postdoc?
(There are similar questions on this site, but those concern Master students requesting letters from a postdoc when applying to a PhD position. Since a postdoc is still above a PhD student, the context is different, so the answers there don't necessarily apply here.)
EDIT: I'm applying for postdocs in Europe and North America.