There’s no reason to face that question until it arises.
Williams still has to face live hitters before going on rehab. Then he’ll have to pitch a few games there before being brought back. And that’s if everything goes well, which is never guaranteed.
When he is brought back, it would make sense to have his first appearance or two for the Brewers be in lower leverage situations.
After all of that is done, then see where things are. Maybe the best thing will be to not have a designated closer. I certainly wouldn’t assume that Williams will be ready to go on consecutive days or 3 times in 4 or 5 days immediately or that he will be as sharp as he has been in the past.