The screw on the top right? Then I take it that it is not the one with the L seal, which is the top screw--the black one. I think it would weaken the mount. whether that is serious would depend on how large and heavy the lens is. If it's a 35mm Summaron, that's one thing. A 90mm Summicron, well, then I would worry.
But just as importantly, how could this screw get broken off? Is the head broken off? I would worry that it could mean the camera, with lens attached, had a bad accident. And that makes me worry that the critical lens-to film-plane distance may have been distorted.
Do you know this camera's history?
“There is hardly anything in the world that some man cannot make a little worse and sell a little cheaper, and the people who consider price only are this man's lawful prey.”