In a corresponding move, the Brewers activated relief pitcher John Axford to the major league roster, hours after acquiring the Canadian from the Toronto Blue Jays.
The Brewers had no immediate comment on whether Hader is vaccinated or experiencing any symptoms of COVID-19. He will be required to miss a minimum of 10 days while on the IL.
The 27-year-old is tied for eighth in the majors with 22 saves this season while pitching to a career-best 1.83 ERA. He pitched a perfect shutout ninth inning on Sunday to preserve a one-run win over the Atlanta Braves.
The Brewers enter play Monday with a 63-43 record to lead the NL Central.