He's replacing Cameron who went on the paternity list. Replacing a player who goes on the IL/paternity/bereavement voids the required 15 days for a pitcher to be in the minors after being optioned.
Personally I think it's dumb that you can use a position player going on one of those lists to bring back up a pitcher, but it's a loophole that the Brewers have used quite a bit the last couple of years.