See, the thing is, according to some background dialogue dug up by Comic Book, Joey is the guardian angel who has been keeping all of the Walkers at bay.
Back when Jesus and Carl were hitchhiking in the back of the Saviors truck, we overheard a couple of Saviors talking about the Walkers. Well it turns out that there is a MASSIVE herd nearby, but the reason that they haven't been a threat at all for a while is that the Saviors have figured out how to control the herd and keep it away from communities. Scratch that, JOEY had figured it out, the Saviors name him as the one behind the 'redirect' and that he had been using a clever string of explosions to lead the Walkers away.
When the truck gets to Sanctuary we hear Negan outside talking to Fat Joey and we hear that something has gone wrong with the redirect.
It appears that Joey is the only one who knew how to keep this redirect up and how to fix the problem, it might even be why he was choosing to let Daryl go. He knows how important his role is to EVERYONE.
Chances are there is now a herd of Walkers out there that is completely out of control.
Now we know Negan isn't the most emotional man, but chances are he is going to be upset about this one and the implications it has.
One things for sure, we're going to be seeing more Walkers in the second half of this season.