As good as things are going for the D-backs these days, there still has to be at least a little interorganizational regret over "The One Who Got Away" -- the guy they dealt in his mid-20s who has torn it up in the early portion of the season with a beautiful blend of power, production and defensive prowess.
Of course, every word above applies to Didi Gregorius, your odds-on favorite for the American League Player of the Month Award. But Gregorius, who was traded from Arizona prior to the 2015 season, plays for the New York Yankees, and people tend to pay a considerable amount of attention to the Yankees. Let's go out on a limb and assume you've noticed Gregorius' sizzling start. 
You might not know as much -- or anything, really -- about Mariners right fielder Mitch Haniger, who, like Gregorius, has exceeded all reasonable expectations since he was shipped from the Snakes. He is the best hitter in baseball that nobody's talking about.
So let's rectify that. 
Haniger, a 27-year-old who is in his second full season, entered the week tied for the Major League lead in home runs, with 10. He drove in 27 runs in March/April -- the most for any Seattle player not named Ken Griffey Jr. Five of his Haniger's RBIs were of the game-winning variety, tied for the most of any player in the Majors in the opening month-plus. His weighted runs created plus (wRC+) mark (186) trails only that of Gregorius, Mookie Betts and Manny Machado among players with at least 100 plate appearances.

In an opening month in which the Mariners were trying to survive injuries to Nelson Cruz, Ben Gamel, Ryon Healy and Mike Zunino, a healthy Haniger was especially essential. Seattle has posted an above-average offensive output (it's 109 team wRC+ ranked seventh in the Majors) and a strong 16-11 record in the daunting AL West, and the hard-swinging Haniger is their early MVP.
"He doesn't hold back," M's skipper Scott Servais said. "When he does swing, he lets it rip. He's been outstanding getting his pitch to hit and not missing it."
Yeah, Haniger doesn't get cheated. His swings hard, he connects hard. Haniger's 52.05-percent hard-hit rate (percentage of batted balls with an exit velocity of 95 mph or more, per Statcast™) ranks 11th in MLB. But he is not totally selling out to produce his power. Haniger's 10.7-percent walk rate is a 3.1-percent rise from his 2017 rate. So he's learning to take his walks, as well.
"When he's sound mechanically and his timing is good, he can cover the plate, up and down," hitting coach Edgar Martinez said. "That's what it's been this year. He's been very consistent with his mechanics."

Haniger changed his swing mechanics in 2015. Coming off a year in which his power output (11 homers and 11 doubles in 282 at-bats, primarily at Double-A) was decidedly average ("I was stalling out," he says now), he began to incorporate a leg kick and studied the right-handed swings of Josh Donaldson, Buster Posey, Jose Cabrera and Ryan Zimmerman, among others. The new look helped him begin to drive the ball to the opposite field with more authority.
"I was in a place where I had nothing to lose," Haniger said. "In the past, I couldn't hit the ball to right field or right-center. It was just a swing issue, it wasn't an approach issue."
After making the change, Haniger hit 38 homers over the next two Minor League seasons. Beyond that pure power, it was his scrutiny of his own failings and his high-IQ adaptation to a new style that drew the attention of general manager Jerry Dipoto when the Mariners targeted Haniger in the Jean Segura-Taijuan Walker deal prior to 2017.
Dipoto himself looked like a high-IQ exec last April, when Haniger stormed out to a .342/.447/.608 slash line in his first 21 games with Seattle. But injury issues -- an oblique strain in April, then a fastball to the face in July -- clouded an otherwise impressive rookie year.

If Haniger merely repeats his rookie numbers (.843 OPS and 129 wRC+, with seven defensive runs saved in the field) this year, that would qualify as quality output. But like so many others in the midst of MLB's so-called "air-ball revolution," he's made a more conscious effort to elevate this year. And Haniger's reduced ground-ball rate -- from 44.0 last year to 32.9 so far this season -- could push his offensive ceiling higher, as we've seen so far.
"I'm not really trying to hit a fly ball," he said. "I like to think about trying to hit a line drive over the center fielder's head… I know some guys around the league after at-bats will go look at what their exit velocity and launch angle were. For me, I know when I hit the ball hard, and when it's a line drive, it's going to be a good launch angle. So I don't need to go look at data for that. Trying to backspin the ball to dead center field is the big focus."
Here's what separates Haniger right now: At a time when pitchers are actively trying to combat the uppercut swings by doing some elevating of their own -- throwing more pitches up in the zone -- Haniger's swing has been versatile enough to account for everything. On pitches tracked by Statcast™ in either the upper-third of the strike zone or the upper-half outside the zone, Haniger's 1.074 slugging percentage was, as of this writing, tops among those with at least 25 results.

"You get it up in the zone and he can reach it, man," Indians manager Terry Francona said. "He's getting dangerous."
We saw that on Haniger's 10th homer of the season Sunday, when Tribe reliever Zach McAllister challenged him with a four-seamer in the upper-third of the zone and Haniger deposited it into the left-field bleachers.
"I used to be really bad at hitting the high strike," Haniger said. "I kind of made some swing changes to make my swing more adjustable to where if it's up, I can get there. And if it's down, I can get there. I'm trying to cover every quadrant of the zone."
The Mariners have uncovered a hidden gem in Haniger. He doesn't get the coverage a guy like Gregorius does, but the outfielder been every bit as impactful on his club's fortunes in the early going in 2018.