ANAHEIM -- It was the kind of night where it seemed like nothing went right for the Angels.

Left-hander Andrew Heaney gave up four runs in the first inning, Justin Upton exited after the second with lower back tightness and the offense scuffled in a 5-0 loss to the Giants in the series opener at Angel Stadium. It snapped a streak of the Angels scoring at least three runs in 16 straight games.

Heaney, though, did settle down after his shaky start to his outing, getting through six innings with 10 strikeouts, not allowing a run after the second frame. But he put the Angels in too big of a hole early and they couldn't recover.

"Ultimately, we lost the game, so I'm not terribly happy, but it's probably the best I'll feel about giving up five earned in a game," Heaney said. "There's probably a lot to like about it, but there's obviously a lot not to like about it. It's one of those ones that I just want to go back and figure out exactly what was going on in the first inning versus the rest of the game."

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Heaney wasn't helped by his defense in the first inning, either. After giving up a one-out single to Mike Yastrzemski, Darin Ruf smacked a liner to right-center that was misplayed by right fielder Luis Rengifo for a double. It proved costly, as Buster Posey followed with an RBI groundout before Brandon Belt dropped a perfect RBI bunt single down the third-base line.

Heaney then surrendered a two-run blast to Wilmer Flores on a first-pitch fastball to make it a four-run game. Mauricio Dubón also jumped on the first pitch of the second inning, depositing a fastball from Heaney over the center-field fence to give the Giants a five-run lead.

"Giving up the two-run homer was the backbreaker there,” Heaney said. “The two homers they hit out were just middle-middle heaters on the first pitch. I understand that teams know that I throw a lot of fastballs and they're going to be aggressive. When I put it in the right spot, good things happen. But I'm no different than any other guy in this league where if you throw balls down the middle, it generally doesn't end up well."

The Angels threatened against Anthony DeSclafani in the second with runners at second and third and nobody out. But they failed to score and Upton injured himself while striking out to end the inning. He was diagnosed with lower back tightness and was replaced by Juan Lagares.

Upton felt the back stiffness as the game went on and didn’t hurt himself on any one swing but is continuing to be evaluated. There’s a chance it could affect two-way star Shohei Ohtani’s status, as he was expected to hit for himself as the pitcher Wednesday, but they could decide to be conservative and not lose the designated hitter if Upton isn’t available to play.

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"This is one of those things where you probably have to wait till tomorrow at least to figure out how severe it is,” manager Joe Maddon said of Upton. “It was bothersome and I had to take him out immediately. They started to treat it, but we should know more by tomorrow morning."

The offense was held in check by DeSclafani after the second as they managed just six hits on the night, while going 0-for-5 with runners in scoring position. Two of those hits didn’t come until the ninth inning. The Angels were held scoreless for the fourth time this year.

“That's one of those where I hate to say tip your hat, but he had everything going on,” Maddon said. “We had some opportunities early. We had a second and third nobody out with Rengifo, [David] Fletcher and J-Up and nothing happened. We didn't get him early, and if we had, it obviously would have been a different ballgame. But he keeps getting better. You saw the slider, the fastball, the movement on the fastball, a couple changeups when he wanted it. He just really pitched well. So did their bullpen.”