Ryu was placed on the DL with a strained right glute muscle and will miss the minimum 15 days. His return, coming one day before rosters expand, will require a corresponding roster move. Kevin Correia will move from the starting rotation to long relief.
Mattingly said as long as Ryu doesn't experience a setback, the decision makes sense because of matchups. Ryu has allowed the Padres one run in 19 innings this year. Hernandez has allowed Washington, Monday's opponent, one unearned run in 15 innings this year.
Mattingly also said that Scott Van Slyke's turned right ankle is better than expected. He took batting practice with the team, was available as a pinch-hitter and could start Sunday against left-hander Eric Stults.
Juan Uribe said his hamstring felt fine after Wednesday's running test. The third baseman is eligible to be activated Sunday, but Mattingly hinted the club might wait until Monday, when rosters expand.
Ken Gurnick is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.