Most cameras need both lens and mount to be sealed for the true resistance. However, ide be more concerned about the lens itself getting soaked than water getting in from the mount. There is a lot less area for exposure on the mount compared to the lens sticking out with various points for entry. Focus rings, zoom rings, etc...
It also depends on the amount of rain too. I've shot plenty of non sealed gear in light drizzle and been fine for a few moments.
If you plan on shooting through a storm ide have both sealed. If it's just a sprinkle while making that last shot before the camera goes in the bag.. a non sealed lens would be fine.