The same way you avoid it: Succession planning. You need to do the same thing as before this becomes an issue, but now you have less time. You need/want battle plans before a battle, but if you don't have them before you have to make them during.

I don't think there's a contextualess answer, but broadly you either find a direct replacement that you mentor (or maybe you're lucky and they're already GTG) or you diminish your role and have multiple other roles within your org (or outside, i.e. outsource) take up what used to be your responsibilities until your role is obsoleted.

Probably try to hire someone more senior, preferably with management, sales or bizdev experience. And then start pairing with them until they have his specific workflow down. It may never be anyone as good as he was but the alternative is that the business goes under when he can no longer work.