Somsen was called up to the majors for the first time last month and spent three days in Cincinnati without appearing in a game. He was a 22nd-round pick in the June 2013 draft.
Cincinnati's bullpen has given up at least one run in 25 of the last 26 games. It's last in the majors with a 6.59 ERA and 27 homers and 63 walks allowed in 32 games.
The Reds optioned outfielder Kyle Waldrop to Louisville on Monday to open a roster spot. He was called up Sunday but didn't play against the Brewers.