First Chapter Without the MC an Automatic Turnoff?

I wouldn't think so- I can name several published books off the top of my head that do not start with the protag- with my own books only one of the four that I have published start with the protag and that is because that one is written in first person. The current WIP I am writing starts with the antogonist- you don't meet the protag until chapter two.

As long as the 1st chapter is necessary to the story, what difference does it make?

I hope not. The novel I'm currently querying doesn't start with the protagonist. It starts with the villian. My understanding has always been that the most important thing about the first chapter is that it be intriguing enough to get people to want to read chapter two, no matter which characters are featured in it.