The bulk of Del Bosque's squad for this summer's World Cup will be made up of players from Barca and Los Blancos, who have clashed in previous meetings.
"There's going to be tension," Inside Spanish Football quotes the Spaniard as saying.
"I'd like to see a good game, without too many fouls because it isn't in our best interest. The World Cup is right around the corner.
"The players have to fight for their clubs, but they have to behave on the pitch. Any unsportsmanlike conduct will reflect poorly on the players."
Del Bosque names his final squad for the World Cup next month.