The team promoted Sam Tuivailala from Triple-A Memphis to fill Siegrist's spot on the roster.
Siegrist, who led the majors with 81 appearances last year, is 5-2 with a 2.78 ERA and 0.96 WHIP in 35 games this season. He had been feeling ill since June 16, but he only recently got the diagnosis that he has mono. The DL stint is retroactive to Thursday, and the team expects Siegrist to return shortly after the All-Star break.
"We sent him home, and the more he can relax and rest, the quicker he's going to get over this," manager Mike Matheny said. "We just need him away from here and off his feet."
Tuivailala, 23, is a converted infielder who was 2-1 with a 5.06 ERA for Memphis.