Mattingly said Guerrero will DH if he doesn't start in the field in order to maximize his playing time. He sat out the 2013 season before defecting from Cuba, and the rust shows.
Brendan Harris, a non-roster invitee competing for a utility role, singled home the go-ahead run.
The club had to dip deep into the bullpen after Zack Greinke left in the first inning with a mild calf strain.
Chris Withrow followed Greinke and allowed a pair of runs with three walks, but prospect Red Patterson struck out two in 2 1/3 perfect innings, Sam Demel pitched a scoreless inning and Daniel Moskos struck out all three batters he faced. Moskos, a lefty, is a former first-round Draft pick who finished last season pitching in Taiwan.
Pedro Baez got the win, Jarret Martin the hold and Carlos Frias the save with an inning each.
Ken Gurnick is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.