OAKLAND -- 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.

Oakland Athletics’ Brandon Moss (37) follows the flight of his grand slam off Los Angeles Angels starting pitcher Garrett Richards (43) as catcher
Oakland Athletics' Brandon Moss (37) follows the flight of his grand slam off Los Angeles Angels starting pitcher Garrett Richards (43) as catcher Chris Iannetta (17) looks on in the first inning of their MLB game at O.co Coliseum in Oakland, Calif., on Friday, May 30, 2014. Coco Crisp (4), John Jaso (5) and Josh Donaldson scored on the grand slam. (Ray Chavez/Bay Area News Group) ( RAY CHAVEZ )

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.

Oakland Athletics’ Brandon Moss (37) is welcomed home by Coco Crisp (4) and Josh Donaldson (20) after circling the bases on his grand slam off Los
Oakland Athletics' Brandon Moss (37) is welcomed home by Coco Crisp (4) and Josh Donaldson (20) after circling the bases on his grand slam off Los Angeles Angels starting pitcher Garrett Richards (43) in the first inning of their MLB game at O.co Coliseum in Oakland, Calif., on Friday, May 30, 2014. John Jaso (5) also scored on the grand slam. ( Ray Chavez/Bay Area News Group) ( RAY CHAVEZ )

The offensive largess was manufactured on behalf of Drew Pomeranz, the left-hander making his fifth start after being elevated from the bullpen. He left the game one out into the sixth inning with a 9-3 lead, but the two runners he had left on base came around to score. The five runs that Pomeranz (5-2) was charged with came after he had allowed just two runs in his first four starts.

"This team is always doing a good job producing runs," Pomeranz said after his longest start of the season. "It was good to see Moss get us going. It seems like he's always in the middle of it."

Melvin would like to see Pomeranz get deeper in to games as he becomes a more nuanced member of the A's rotation. So far his starts have lasted five, five, five, four and 51/3 innings. That leaves more outs for the bullpen to gobble up, and Melvin doesn't want to put any more strain on the relievers than necessary.

  • Eric O'Flaherty (elbow surgery) will pitch Monday in extended spring training in Arizona. After that, he's likely to be scheduled for a 20-day injury rehabilitation. That would mean the reliever is on course to be in the A's bullpen in three weeks or less. "Physically I feel really great," he said. "I'm just anxious to know what we're going to do next."

  • Ryan Cook (strained forearm) said he's hoping to be on the plane to New York when the A's take off for their next road trip Monday. Cook is scheduled to throw at Single-A Stockton on Saturday in the first game of an injury rehab assignment. Because the A's have been monitoring Cook's work and because he's faced hitters, there is some belief the one game may be all the tune-up he needs.

  • Eric Sogard got Friday's start off thanks in large part to his .158 average in May. Alberto Callaspo was in the lineup. Sogard, batting .188 overall, is like right fielder Josh Reddick in that he gets some extra at-bats because the A's like his work on defense. But Melvin said he thought the second baseman needed a break, although he did take over from Callaspo on defense.

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    SATURDAY'S GAME
    L.A. Angels (Tyler Skaggs
    4-2) at A's (Tommy Milone 3-3), 7:05 p.m. CSNCA