SEATTLE -- For four innings, rookie Justus Sheffield looked for all the world like the promising pitching prospect the Mariners are counting on as part of their future plans.

But then the world shifted, Sheffield wobbled, his bullpen collapsed and the Mariners were looking at an 11-1 loss to the A’s on Monday in the series finale at T-Mobile Park.

• Box score

Oakland erupted for eight runs in the fifth off Sheffield and veteran reliever Bryan Shaw, who was claimed off waivers by Seattle just prior to the season opener 11 days earlier. Shaw doesn’t figure as part of the long-term future, but Sheffield remains an integral piece going forward and will look to build on the early start and learn from the rocky fifth.

“That’s the Sheff we’ve seen,” manager Scott Servais said of the first four frames. “He gets on a roll and gets the breaking ball going. They had a game plan and wanted him to get the ball up because he lives at the bottom of the strike zone. I think he recognized that after the first half-dozen guys and made a good adjustment to keep the game rolling.

“It’s a learning process for him. There’s a lot of upside. I really like Sheff, and he got a lot out of tonight’s game. Not the ‘W’ he was hoping for or getting through the fifth, but he’s still going to learn a lot from it.”

The 24-year-old’s final line was four hits and four runs over 4 2/3 innings, with two walks and five strikeouts as he fell to 0-2 with a 9.39 ERA.

“Honestly, I liked the way I was throwing the ball,” Sheffield said. “I felt I was out there attacking those guys and going right at ‘em. It just didn’t turn out the way I wanted.”

Working with a sharp slider and 92-93 mph fastball, Sheffield allowed just a single and one walk through four scoreless innings and took a 1-0 lead into the fifth. He then retired the first batter and had an 0-2 count on Khris Davis before things went sideways.

Davis worked his way back for a nine-pitch walk, and Stephen Piscotty and Sean Murphy followed with singles to load the bases. Servais tried to let Sheffield work his way through the jam, and he struck out Marcus Semien for the second out, but then the rookie gave up a two-run single to Ramón Laureano.

That brought Shaw out of the bullpen, and the former Indians standout allowed the next seven batters to reach base on four singles, a double and two walks before Semien flied out to end the parade.

“Bryan is struggling, no question,” Servais said. “He’s had a ton of experience and a lot of success in this league. The last couple years have been rough for him over in Colorado, and we’re trying to give him some information and things that might help him get the results he’s looking for.”

Zac Grotz and Nestor Cortes worked the final four innings for Seattle and -- combined with Shaw -- the trio wound up allowing seven runs on eight hits, nine walks and three hit batters. Things could have been worse, but the A’s stranded 13 runners on the night.

With 11 walks issued overall, the Mariners now lead the Majors with 56 in 11 games. “Not a fun game to watch, by any standards,” Servais said. “We walked a lot of guys and did not control the strike zone. Unfortunately, it got ugly there. Nothing we can do about this one. We have to get in the shower, wash it away. We’ve got the Angels coming in and need to get back on track tomorrow.”

Kyle Seager provided the lone offense for Seattle as he went 2-for-3 with a walk and notched his team-leading 11th RBI on a third-inning double. Rookie center fielder Kyle Lewis saw his 10-game hitting streak snapped with an 0-for-4 as the Mariners totaled just four hits in losing their third straight.