I made a post yesterday regarding when I should send a professor a reminder regarding a LoR request. Well, he actually got back to me today and told me that (translated and paraphrased from Korean):
"(small talk) I think it would be best for you to ask someone who may know you better. I'm afraid I may not know you well enough to write a detailed and strong recommendation letter. However, if you can't find anyone else I'd be glad to write one.
"Since I don't know you that well, it would help if you sent me your CV, SOP, and a one-page document laying out what you want me to specify or emphasize.
"Again, I strongly advise you to request a letter from a professor who knows you well, as that would lead to a stronger letter."
So yeah, it's not exactly an ideal response, but was expected. The problem I had is that I'm a MSCS student but am not originally from CS. Being busy with my own advisor and lab/projects has somewhat hindered me from becoming "close" with other professors. This particular professor I asked is one of the three who I've had interactions with a little more than the classroom level (I very briefly worked on a project with his lab mates, but it didn't turn into anything). The other two have stated they would write me letters already.
There is a postdoc in our lab who I've worked with, but I'd rather not ask a postdoc.
How negatively is this going to affect me? I don't think I really have a choice, and I don't believe this professor is someone who would write anything negatively about me, but I guess I would like some perspective from people who have been on both sides. Thanks.