Relief pitching hasn't been among the Padres' strengths this season. That group entered Saturday ranked 24th in Major League Baseball with a 5.35 ERA.
Closer Kirby Yates is likely out for the season because of an elbow injury that required surgery earlier this week. Drew Pomeranz is on the injured list with a shoulder strain.
Rosenthal has had a terrific rebound year after posting a 13.50 ERA in 22 appearances with the Washington Nationals and Detroit Tigers last season. The 30-year-old put up a 3.29 ERA with nine hits allowed and 21 strikeouts in 13.2 innings for the Royals.