The A's lost their top run producer Friday when Brandon Moss left a 9-5 win over the Los Angeles Angels with a bit of right-calf tightness.

The good news for the A's is that he shouldn't miss more than a game or two. And that's good news for Josh Donaldson, the team's other major RBI threat who hit two home runs and drove in four runs to augment Moss's first-inning grand slam Friday.

Moss has been hitting behind Donaldson for most of the season, and that's meant that Donaldson has seen better pitches to hit. Such was the case in the second and third innings when Donaldson homered, then delivered an RBI single with Moss in the on-deck circle.

But the tone of the game was set in the first inning when Donaldson, batting with men at the corners, drew a four-pitch walk from Angels starter Garrett Richards.

"Richards is going to be tough on a right-handed batter like me in that situation," Donaldson said. "So it's my job to get on base, because we flip the switch with (the left-handed) Moss up."

Moss looked at two pitches, both balls, then launched his 13th homer of the season into the right field seats for his first major league grand slam.

"Right there he (Richards) needs to throw a strike," Moss said. "I was able to get one I was looking for."

Things were fine for the A's, who had a 6-0 lead after the Donaldson homer in the second inning. But when Moss took the field to start the third, he felt something odd. In the bottom of the inning, after Donaldson's single made it 7-0 Kyle Blanks pinch-hit for Moss.

"There was cramping in my right leg," Moss said. "I tried to get rid of it, but it wouldn't go away. After the inning, I told (the training staff). It would have been pretty difficult to go on, because it was really hurting. I'm all right now, but it still hurts some."

Manager Bob Melvin said Moss would not play Saturday, a decision made all the easier by the fact the Angels are throwing a left-handed pitcher in Tyler Skaggs and it'll be an easy thing to get Blanks, his right-handed first baseman, in the lineup. The bigger question will be Sunday when Moss figures to be a major weapon against Jered Weaver, the Angels' right-handed ace.

Melvin's history is to err on the side of caution, and Moss himself said he had no idea if he'd be ready by Sunday. But the A's are treating the injury as a day-to-day thing. They want Moss, who has a .276 batting average and a team-best 46 RBIs, available long term for a 33-22 team that leads the American League West by 2½ games over the Angels.