D-backs right-hander Zac Gallen was a third-round Draft pick in 2016, has been a member of three franchises already and never earned a spot on any organization’s preseason top-10 prospects list. But since he arrived in the Majors in June 2019, few starting pitchers have been better.

In his most recent start, Gallen threw seven innings, struck out seven, walked none and allowed two runs against the Rockies, in the process becoming the first National Leaguer to begin his career by making 19 straight starts without yielding more than three earned runs in any of them. Gallen will look to keep his streak going Monday night against the red-hot A's. The MLB record belongs to Aaron Sele, who allowed three earned runs or fewer in 21 straight starts to begin his career with the Red Sox in 1993-94.

Gallen made his MLB debut with the Marlins on June 20, 2019, a little over a year after the Cardinals shipped him to Miami in the Marcell Ozuna trade. Seven strong starts into his Marlins tenure, he was traded again, this time to the D-backs for shortstop prospect Jazz Chisholm (MLB's current No. 72 prospect). Gallen continued to excel with Arizona, recording a 2.89 ERA with 53 strikeouts in 43 2/3 innings, and it has been more of the same in 2020.

Since Gallen debuted, only seven starters have recorded a better ERA, and his 29% strikeout rate is tied for 19th best among the 104 pitchers to make at least 15 starts in that span.

Lowest ERA since June 20, 2019
Min. 15 starts
1. Jack Flaherty -- 1.76
2. Gerrit Cole -- 1.86
3. Jacob deGrom -- 1.87
4. Sonny Gray -- 2.21
5. Shane Bieber -- 2.52
6. Justin Verlander -- 2.60
7. Mike Clevinger -- 2.79
8-T. Zac Gallen -- 2.80
8-T. Lance Lynn -- 2.80
10. Zack Greinke -- 2.88

Here’s why the 25-year-old’s initial success could be sustainable. (All stats below are through Saturday.)

Despite average velo, his fastball has given hitters fits

Gallen doesn’t throw especially hard, averaging 92.9 mph with his four-seam fastball -- right around the MLB average (93.3 mph in 2020) -- but the pitch has been effective for him nonetheless.

In 142 career at-bats ending on a four-seamer, he has held opposing batters to a .190 average and a .289 slugging percentage, with superb expected stats (.218 xBA, .369 xSLG) behind those numbers.

Gallen throws the pitch with above-average spin (2,373 rpm), which has helped him generate a solid lifetime 23.3% whiff rate (misses/swings) on four-seamers.

He also frequently sneaks the pitch by batters for called strikes. The righty has thrown 793 four-seam fastballs in his career, and 196 (24.7%) of them have resulted in a called strike, one of the highest rates in MLB.

Highest % of called strikes on four-seamers, since 2019
Min. 750 four-seamers
1. Zack Greinke -- 28.9%
2. Tommy Milone -- 28.0%
3. Shane Bieber -- 26.4%
4. Zac Gallen -- 24.7%
5. Clayton Kershaw -- 24.2%

In general, Gallen’s four-seamer doesn’t get put in play all that often. Just 91 of his 793 four-seam fastballs (11.5%) have been put in play, the third-lowest rate among pitchers with at least 750 four-seamers thrown since the beginning of 2019.

When batters have put his four-seamer in play, they’ve recorded a barrel -- batted balls with an ideal combination of exit velocity and launch angle for producing extra-base hits and homers -- just 5.5% of the time (five total barrels/91 batted balls). The league average barrel rate on four-seam fastballs in 2020 is 9.2%.

Avoiding the middle of the zone has been key for Gallen in preventing hitters from hammering his four-seam fastball. The righty has thrown 67.7% of his four-seamers either in the shadow or chase zones, while only 27.4% have ended up in the heart of the plate.

He has a diverse repertoire

In addition to locating well, Gallen prevents hitters from sitting on his fastball by mixing up his pitches. The right-hander’s arsenal also includes a curveball, cutter and changeup, and he not only throws his four pitches regularly but also consistently gets whiffs with all of them. Here’s his career whiff rate on each of his pitches, with the MLB average since 2019 for each offering included in parentheses.

4-seamer: 23.3% (21.8%)
Curveball: 35.3% (31.8%)
Cutter: 27.6% (25.1%)
Changeup: 38.2% (30.9%)

Gallen is one of nine pitchers across MLB to post a 23% whiff rate or better on four different pitch types (min. 100 swings on each) since the start of 2019. The list is star-studded.

Pitchers with 23%+ whiff rate on 4 pitch types, since 2019
Min. 100 opponent swings on each pitch type
Chris Bassitt
Trevor Bauer
Walker Buehler
Mike Clevinger
Gerrit Cole
Yu Darvish
Zac Gallen
Max Scherzer
Noah Syndergaard

Like he does with his fastball, Gallen keeps his secondary pitches around the edges of the plate. In 2020, the righty has thrown 48.1% of his pitches in the shadow zone, tied for the seventh highest in MLB.

He neutralizes both righty and lefty hitters

Having such a diverse repertoire has helped Gallen hold both righty and lefty batters in check. In his career, he has limited righty batters to a .678 OPS, and lefties to a .661 mark.

Gallen’s go-to secondary pitch when facing lefties is his changeup, which he has thrown more than a quarter of the time in such matchups, compared to less than 8% of the time against righties.

With the pitch typically drifting down and away from them, southpaw hitters have recorded a .172 average with a .259 slugging percentage and 20 strikeouts in 58 at-bats ending on Gallen’s changeup.

Against righties, Gallen has been able to dominate with his four-seamer and curveball, holding hitters to a combined .152 average and .288 slugging percentage with the two pitches in those matchups.

It's still early, but Gallen seems to have found the recipe to get big league hitters out consistently. And after being dealt twice before the age of 25, the righty now looks like a keeper for the D-backs.