A note of confidence, just because someone on your team quit, doesn't mean there isn't hope. It depends on the position in the game, the team composition, and how good the player who quite was. Most likely if he quit he was doing badly, in other words feeding. By him quitting it means the enemy heroes get fed less, there's less people competing for last hits, and you also get all his gold, meaning that your supports will be getting more money which they will need, and your carries will also benefit.
Of course, you will be a man down, but again, depending on your team you can use these advantages to perhaps turn the game around. It isn't something you can draft for, but don't give up either!