SEATTLE — The Oakland A’s became one of the bigger surprises in baseball last weekend when, after six straight wins, they improved to .500 for the season and moved to within 1 1/2 games of first place in the American League West.

It’s been a bit of a struggle for the A’s ever since. Sunday, they lost 8-4 to the Seattle Mariners at T-Mobile Park, marking their sixth loss in the last eight games.

The A’s trailed 5-0 after two innings, but a Max Schuemann error paved the way to a four-run second for the division-leading Mariners, who took two of three in the series.

After losing three of four to the Texas Rangers early last week, the A’s (19-23) are now 3 1/2 games back of first place in the division going into their four-game series against the Houston Astros. The series starts Monday.

A’s starting pitcher Alex Wood allowed four hits, including a two-run home run to Julio Rodríguez, in two innings before he left the game with a shoulder injury, which he said had been bothering him for a while. A’s manager Mark Kotsay said the team would likely have an announcement on Wood on Monday.

“He’s been grinding,” Kotsay said of Wood. “He hasn’t felt great. He’s been able to make every start, but today, you saw his velo dropping; his slider wasn’t as sharp. He gave us everything he had for two innings. Obviously, he’s disappointed that he had to come out of the game.”

Asked if he might have to land on the injured list, Wood said, “We’ll see how the next few days go. I’ve been throwing with it for a little bit now. Just getting treatment and managing the workload.”

For the Mariners, Julio Rodríguez and Mitch Garver both hit two-run home runs, and Luis Castillo allowed two runs over six strong innings.

“Our offense showed up today. Put good pressure on them early, got some big hits, home runs,” Seattle manager Scott Servais said.

Rodríguez’s homer was just his second of the season and his first at T-Mobile Park in Seattle’s 22nd home game. Rodríguez hit a 2-2 pitch from Wood out to straightaway center field in the second inning for a two-run shot that gave Seattle a 5-0 lead.

The homer had an exit velocity of 109 mph and traveled an estimated 409 feet.

“You’ve got to stay patient and let the results come and I feel like today was the day and I’m really happy about that, that I was able to help the team win,” Rodríguez said.

Rodríguez nearly homered in his next at-bat in the fifth inning, doubling off the top of the wall, but jogged home when Garver hit his fifth of the season to give Seattle a 7-1 lead. Garver also had a two-out RBI single in the first inning.

Seby Zavala added his first home run of the season for the Mariners, a solo shot in the sixth.

Following a sluggish start to the season, Luis Castillo (4-5) has now gone six straight starts allowing two earned runs or fewer each time. Castillo needed 100 pitches to get through six innings, but closed his outing with strikeouts of Shea Langeliers and J.D. Davis with runners on base.

Castillo allowed seven hits and struck out eight. Schuemann and Abraham Toro both hit solo home runs to account for the scoring off Castillo.

Brent Rooker hit his 10th homer of the season in the eighth inning off reliever Cody Bolton. It was a two-run shot.

“These guys battled back. Every time we scored, they ended up scoring. As much as we’ve been able to lock it down in games that we’ve trailed in — and we actually have come back in a few of them — today we just weren’t able to do that in the bullpen,” Kotsay said.

UP NEXT: Ross Stripling (1-6, 5.14) will open a three-game series at Houston on Monday. Stripling gave up 10 hits and 11 runs — but only five earned runs — over 1 2/3 innings in his previous start against Texas.