As the A’s lurch back and forth between good streaks and bad, it becomes ever more evident that this year’s club is probably looking, at best, at a .500 mark and middle-of-the-pack finish.

Inconsistent offense and dreadful defense will make it tough for Oakland to climb back into contention; Houston has emerged as the division’s dominant club and the wild-card teams aren’t likely to come out of the mediocre AL West.

Which means that some point, the A’s will turn toward their future, that group of well-regarded position players at Triple-A Nashville. But they’re not going to rush anything.

“We do have some young players we think very highly of,” vice president of baseball operations Billy Beane said, “but we want to make sure they’re given the benefit of all the development possible so that when they get here, they stay here.”

Expect shortstop Franklin Barreto, 21, to arrive first. He could be up any time from mid-June on, but is more likely to be promoted in July after the A’s begin their usual sell-off of players approaching free agency. Second baseman Jed Lowrie, playing the best he has since 2013, would be a solid addition for a contender, a switch hitter who turns in good at-bats and whose defense, though limited, is much improved from last year.

Everyday shortstop Marcus Semien is expected to come off the disabled list as soon as mid-June and second base is the spot most scouts see Barreto playing, anyway. So, barring injuries, that’s the scenario to expect: Lowrie dealt, Barreto up.

“I’m impressed with how well (Barreto) is starting to turn on pitches. He’s very aggressive on the bases and he’s consistent defensively,” one National League scout said. “I still see him as an offensive second baseman — Oakland will be much better with him at second in the second half.”

Barreto, acquired in the Josh Donaldson deal with Toronto in November 2014, is batting .314 with six homers and 27 RBIs in 41 games.

“Ability-wise, he’s ready,” an American League scout said. “But he’s a second baseman for me. If he’s there, he has a chance to be an All-Star.”

What of the other top prospects at Triple-A Nashville?

Matt Chapman, 24, is a superior defensive third baseman, but his bat hasn’t quite caught up with his glove. The A’s wouldn’t mind giving him more time with the Sounds, but if there’s a market for third baseman Trevor Plouffe in July, Chapman could be up earlier than anticipated. He is batting .238 with nine homers and 16 RBIs in 28 games after missing time early in the season with a wrist injury.

“He’s a Gold Glove defender,” the AL scout said. “He has that Nolan Arenado-Manny Machado skill set.”

Corner infielder Renato Nuñez, 23, has hit 12 homers in 39 games. , but like Chad Pinder, he doesn’t have a settled position. Pinder — who hit three homers in the A’s four-game weekend series with Boston — already has the big-league utility job, at least until Semien’s return.

“Nuñez is an enigma,” the AL scout said. “Big-time power to all fields but he doesn’t have discipline at the plate and they don’t know where to play him. He’ll smash balls over the fence but he doesn’t have very good at-bats.”

First baseman/right fielder Matt Olson, 23, was up in September and briefly in April. The A’s love his glove and his patience and power at the plate. His high strikeout numbers, not so much.

“I hate to make this comparison, but he’s like Adam Dunn,” the AL scout said. “He controls the strike zone, he walks a ton, he will strike out — and he has big power. It will disappear into the night. He has timing issues, kind of a long swing, but he’s defensively versatile. “

It’s hard to see how the team could wedge Olson onto the roster anytime soon because outfielder Matt Joyce is on a two-year deal and might not draw much interest in July after his slow start. And the A’s have too many first basemen on the big-league roster; should Yonder Alonso get traded in July — not a given, Oakland thinks so highly of him — Ryon Healy probably gets the bulk of the playing time there.

The trickiest one: Bruce Maxwell, 26. He has been up twice, he’s the best catcher in the upper levels of the organization and he showed in September that he can hit at the big-league level.

“It’s hard not to like this guy,” the AL scout said. “I’ve never seen a kid work as hard. He’s always the first one on the field. He’s forced himself into the mix.”

For now, though, he is blocked by Stephen Vogt, who is also a lefty-hitting catcher. Would the A’s consider moving Vogt, the unquestioned team leader? He can play a little first, a little outfield. Were Alonso moved and Healy placed at first, Vogt could hop around from catcher to designated hitter to first as needed, and the A’s can bring up Maxwell.

Maxwell is on Nashville’s disabled list, . He strained an oblique muscle in an at-bat at Fesno last week and will be reevaluated when the Sounds get back to Nashville on Tuesday.

Susan Slusser is a San Francisco Chronicle staff writer. Email: sslusser@sfchronicle.com Twitter: @susanslusser