You can bring this up in Session 0, but I know you're past that now. I would ask your players if they feel comfortable playing another character sheet when someone's absent. Some people like to, some people don't.
In my group, most people don't, because they are too focused on their own character. So we just handwave it narratively, and say that the missing character just fades into the background. They are still there as far as the narrative is concerned, they just don't contribute to fights or roleplaying. When the player returns, it is like his/her character was always there. But you do have to be careful when your players are in a big, dangerous fight, because missing a player can really handicap them. I'd also bring this up in Session 0 if they go that route, making sure they know that there is a greater chance of disaster and TPKs if they choose to not play the missing player's character.