All bikes have a rubber seal that sits in the case and around the gear leaver, if you take the leaver off you will see it if the seal is in position eg (flush with the side if the case) then it will most likely need to be replaced, however if you are lucky you might find that it has piped out and may be able to get away with just very CAREFULLY pushing it back in so as to make sure it doesn't catch on the edge and tear cos then you would be replacing it anyway