"Busts" is a tough word. People hear or read it and instinctively go on the defensive. "Why do you hate [Player X]???" is usually the first response. The second is usually un-printable on a family website, and things tend to just devolve from there.
So, to be clear: These players are all nearly uniformly good. With maybe one exception, I think all will be worth starting in Fantasy for the remainder of the season, and just as many have bright futures; after all, this is mostly a young, talented group. They're all also, obviously, talented players, as exhibited by their first-half dominance.
I just don't think it will last. At least, not to this level. That's not to say you have to dump them all at a moment's notice. But, if someone in your league is willing to pay a premium, say "Thank you," and move on.
Your team will be better for it.
Josh Bell 1B
WAS Washington • #19 • Age: 29
Bell was one of my favorite sleepers coming into the season, but I can't even take a victory lap — I didn't see this coming. One of the best swing-change success stories, Bell has turned himself into one of the league's premier power hitters, after finishing 2018 with just 12 long balls. And there's been nothing fluky about it; he's among the handful of top players in barrel rate, average exit velocity, hard-hit%, and expected wOBA. I wouldn't be surprised if Bell keeps this up having said all of that, but it's worth saying: All of those batted-ball metrics aren't necessarily predictive. Bell really has been this good so far, but this is such a massive outlier for his career, that some regression has to be factored in, even if he is a new player. Pitchers will adjust (he's been a bit susceptible to offspeed pitches and is seeing them more as the season has progressed), and Bell will inevitably go into a slump. Even if you buy him as a top-five-ish first baseman, selling high still makes sense.
SD San Diego • #23 • Age: 23
It's not quite enough to just point at Tatis' 28.6% strikeout rate and .419 BABIP and scream, "REGRESSION!" But that's certainly where you should start. Tatis consistently ran up high BABIPs in the minors but was typically in the .340-.360 range — right near the upper threshold of what major-leaguers can sustain in the long run. So, even if we accept Tatis as an elite BABIP hitter, there's a good ~.050 points of batting average regression coming. When you look into Tatis' underlying skill set right now, what you see is an incredibly talented hitter with consistency issues; about what you'd expect from a 20-year-old rookie. When he squares it up, Tatis can put a hurting on the ball, but he swings-and-misses too often or gets on top of the ball and pounds it into the ground. Tatis is an unbelievably talented player, one you'll probably be drafted in the first two rounds of drafts for the next decade. But he's also in for tougher days in the second half, making him an obvious sell-high candidate. The good news? You already banked his production so far, and now you get to move him for another elite hitter. Go see if whoever in your league has J.D. Martinez is getting antsy.
Mike Soroka SP
ATL Atlanta • #40 • Age: 24
I'm ready to declare Soroka a definitively good pitcher, even though we're just 115 innings into his major-league career. He brings two elite tools to the table, with his 57.0% ground-ball rate and 5.8% walk rate. You can go a long way with great command and a heavy sinker, even in this juiced ball era. However, it's awfully tough to be an ace that way. Dallas Keuchel managed it for a season or two, but he was an even more extreme ground-ball pitcher than Soroka at his peak. Pitching for a good team helps Soroka's value, but with a K/9 below 7.5, he'll have to maintain an ERA in the 3.00 range or lower to truly qualify as a top line Fantasy pitcher. Given his 4.02 SIERA, it's more likely he's in the mid-to-high-3.00s range, making him more of a No. 3 or 4 Fantasy starter.
CIN Cincinnati • #58 • Age: 29
This is the breakout we've all been waiting for from Castillo, right? After last year's surprising dip in strikeout rate, he's put up a 28.8% rate, while his swinging strike rate has spiked to 14.9%. Combine it with a 56.3% groundball rate, and Castillo's a bona fide ace, right? Not necessarily. Castillo is certainly improved from last season, but he's also an obvious regression. One big reason is his control, as his walk rate has nearly doubled from last season, up to 12.3%. Another reason is his strand rate, which sits at 83.2%. Castillo has been outstanding with runners on the bases, cutting his walk rate to 9.9% while allowing just one home run; in each of his previous two seasons, he was significantly worse with runners on than with the bases empty, like most pitchers. Add it all up, and Castillo actually has the worst SIERA of his career at 4.22. I'd take the under on that for his ERA moving forward, but there's a lot of room between a 2.29 mark and, say, a 3.75. If someone in your league views him as a top-10 to-15 starter, this is the perfect time to field offers.
Mike Minor SP
CIN Cincinnati • #31 • Age: 34
Minor might be the single most valuable Fantasy option of the first half of the season, and that's exactly why you should try to trade him right now. Right. Now. Minor just gave you 117 innings of a 2.54 and 1.13 WHIP, but almost none of it was sustainable. Incredibly, Minor actually has a worse SIERA in 2019 than he did last season — though it's slightly better once you account for league context. He has increased his strikeout and ground-ball rates, a good combination, though it's hard to figure out quite how he's pulled it off. His velocity isn't up, and he hasn't introduced a new pitch to keep hitters off balance. More likely, Minor just had a couple of months with uncharacteristically good luck coincide with a hot streak. It happens. It won't happen again.
David Dahl LF
MIL Milwaukee • Age: 28
Coors Field is a great place to play your home games if you're a hitter. In 2019, Rockies' hitters have a .367 BABIP, and that's not all a result of the juiced ball. Over the past decade, that number is .342. Even in that context, it's hard to buy David Dahl's .391 BABIP, and he doesn't exactly do enough to make up for whatever likely regression is coming. If he had a .350 BABIP and nothing else changed this season, his average drops all the way to .278. That's not bad — it's useful, even! — but there's not much here to make him stand out if not for the average. We've been hoping for a season like this from Dahl, but with his regression in power, speed and plate discipline, it's hard to believe this is going to keep up.