graduate admissions - Should I take a "positive, but not enthusiastically strong" letter of recommendation?

I'm currently applying for a Masters degree in a competitive program, and most things on my application are going well.

The problem I'm having is with securing a letter of recommendation - I have already secured two strong letters, one from a workplace manager in the field that I am applying for and another from a professor in an unrelated field. Regarding the third letter, a professor that I spoke to said that he could write a "positive, but not enthusiastically strong" recommendation.

From reading up on answers here and on other sites, it seems that in most cases I should look elsewhere for a letter. However, in my case I do not yet have a letter from a professor in the field that I am applying for. There are a few other professors in the field that I will reach out to, but assuming that they reject my request, should I take this letter?

Another option that I have is to secure a strong third letter from another manager at my workplace. Would taking a third letter from a work manager be better? I am leaning towards this being the case, but I also do not want to jeopardize my application by not having a letter from a professor directly in the field I am applying to.