SAN FRANCISCO -- The first mistake was when left fielder Tyler O’Neill, who was standing underneath an incoming fly ball, thought he heard center fielder Dexter Fowler call for the ball and stepped out of the way, letting the ball drop right next to him to load the bases.

The second mistake was when Miles Mikolas placed an 87 mph slider in the middle of the plate for pinch-hitter Austin Slater to blast over the right-field wall for a grand slam in the fourth inning.

The cost for those two mistakes was the Cardinals’ 8-4 loss Saturday night to the Giants at Oracle Park.

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“I had a bead on the ball, camped underneath it,” O’Neill said. “It was loud in the outfield. But no excuses for it, ball’s got to be caught.”

The fourth-inning mistakes hurt the Cardinals later on, when Paul Goldschmidt hit a three-run home run -- his second home run in as many days -- in the eighth inning to cut the deficit to four. Even though the Giants scored three runs off Tyler Webb in the seventh, had Slater’s ball not gone out, the Cardinals could have taken the lead or been within striking distance late in the game.

“I thought Miles pitched better that what his line showed,” Cardinals manager Mike Shildt said. “Could have been out of the inning, the play didn’t help him out in left, then [Slater] put a swing off it, and off the bat, I was like, ‘OK, sac fly.’ He hit it to the right part of the ballpark.”

The grand slam was the fifth allowed by Cardinals pitchers this season, the most since 2012 when they also allowed five. Only the Giants have allowed more this season with six.

Mikolas has given up two of those grand slams, highlighting a big reason for the right-hander’s struggles this season -- home runs. In 32 starts and 200 2/3 innings last season, Mikolas gave up 16 home runs. In 18 starts and 99 1/3 innings so far this season, Mikolas has matched that total.

Five of Mikolas’ home runs this season have come off his slider, compared to just one last year.

“It’s a pitch I feel like that’s just not doing what I want it to do as much as I’d like,” Mikolas said. “It’s just going down the middle. I’ll check some video, I could be getting my arm up late, maybe I’m overthrowing it, maybe I’m making it do too much.”

After his final start before the All-Star break, Mikolas is 5-9 with a 4.53 ERA. He’s taken a few tough losses -- the Cardinals only gave him four runs of support over five June starts -- and even on Saturday, the wind helped carry Slater’s home run just over the right-field wall.

Mikolas said he might need to adjust the way he’s throwing his breaking balls and hopes to address that over the break.

“The way my year’s been going, it’s just one pitch that gets away from me and I end up paying a lot for it,” Mikolas said. “Could be some mechanical adjustments, I feel like. Maybe I’m just leaving some breaking balls up a little bit more. Got a nice long break here to recharge, reset, take a look at some video and dig deeper into my struggles.”

Wong exits game with calf tightness

Cardinals second baseman Kolten Wong exited Saturday’s game in the eighth inning with left calf tightness after getting hit by a pitch in the fifth inning.

Shildt said Wong will be evaluated Saturday night to see if he will be good for Sunday’s series finale against the Giants.