Mark Trumbo has been a bust since joining the Mariners, exacerbating the club’s OBP problems. / Arizona Diamondbacks

When the Seattle Mariners sent backup C Welington Castillo, reliever Dominic Leone and  prospects to the Arizona Diamondbacks June 3 for DH Mark Trumbo and LHP Vidal Nuno, the trade was panned by those who looked at Trumbo’s on-base percentage and winced.

Sure, Trumbo can hit the ball out of the ballpark, but his OBP had eclipsed the .300 mark just one time in the 29-year-old’s big-league career.

With a long, powerful swing, Trumbo is the all-or-nothing hitter that so rarely has success in Seattle. Plate discipline and a high strikeout rate are his biggest weaknesses.

In other words, he fits right in.

Reaching base at an average clip has eluded every Jack Zduriencik-built roster since he was hired to replace Bill Bavasi.

Since 2009, the GM’s first year at the helm, the Mariners have finished last in the American League in OBP every year, except 2013, when they were 13th. From 2010-12, they were last in the major leagues, a three-year stretch of offensive futility that must have felt like the low point.

Seventy six games into 2015, the 34-42 Mariners are, you guessed it, last in the AL in OBP, at .293. The league average is .314, according to Baseball Reference.

This offense was expected to be much better. After acquiring reigning MLB home-run king Nelson Cruz and setting supposed platoons in both outfield corner spots, manager Lloyd McClendon said the Mariners should score 700 runs this season, a respectable number in an era of defensive shifts and dominant pitching.

If they aren’t the worst offense in baseball, they are the most disappointing. And it doesn’t appear they’ll seek more help from outside the organization before the July 31 non-waiver trade deadline.

“Our thought is the club is pretty much in place,” Zduriencik told The News Tribune last week. “We need to get them to produce. We’ll see what happens. Nobody would ever say no.

“But we brought in (Mark) Trumbo. So now, you look at the lineup, and it’s a formidable lineup.”

Yet the Mariners are on pace to score 544 runs after scoring 634 last season. No one player is to blame. But it’s easy to point to the Trumbo acquisition to expose the flaw of overvaluing power while undervaluing getting on base.

That is the backbone of Oakland GM Billy Beane’s Moneyball theory: Sluggers are overrated, walks aren’t. 

In 20 games since joining the Mariners, Trumbo is hitting .151 (11-for-73) with a .184 OBP, three walks and 21 strikeouts. It’s a small sample, and Trumbo, a serviceable hitter over his career, isn’t likely to continue to look as if he’s swinging with his eyes closed.

There is a human element to Trumbo’s struggles. Having your life uprooted is never easy, regardless of profession or pay scale.

“Getting traded (Arizona) in the off-season was a lot easier than in-season,” Trumbo told The News Tribune. “Guys have to do it all of the time, but it’s not the easiest thing in the world.”

Either way, it’s hard not to second guess the deal. Castillo provided the Mariners a legitimate option to rest an overworked C Mike Zunino (.234 OBP) a day or two per week while Zunino tries to break out of his career-long hitting funk. Backup C Jesus Sucre, hitting .043 (1-for-23), provides no such luxury.

Castillo, meanwhile, is off to a hot start with the Diamondbacks, hitting four homers and driving in 11 runs while posting a .901 OPS in his first 14 games. Because of course he is. That’s what ex-Mariners often do.

The current group is under-performing at an alarming clip. Playing his second year of a 10-year, $240 million deal, 2B Robinson Cano has a .281 OBP with four homers and 24 RBIs.

Left fielder-turned-reserve Dustin Ackley has a .259 OBP to go along with a .199 batting average. CF Austin Jackson is a touch over .300 in the former category while providing little in the way of power (three homers, 13 RBIs) or speed (seven stolen bases in 13 tries). Backup infielder Willie Bloomquist, 37, has a laughable .159/.194/.368 slash line and doesn’t have the arm strength to play shortstop, yet somehow is still on a major-league roster.

The Mariners are tied for fifth in the AL in home runs, thanks in large part to Nelson Cruz’s 19, yet the offense continues to underperform — last in the majors with a .230 batting average, 29th in runs per game (3.36).

Zduriencik is right when he says that players need to start lives up to their career averages. But he also needs to look in the mirror.

Seven years of bad offense is no fluke.