You don't know you're a one-hit wonder when you're in the middle of it. Okay, Right Said Fred or Rednex probably knew the schtick wouldn't work a second time, but typically, you can't identify them until the enough time has passed to know they won't follow it up.

And so it is with Fantasy baseball one-season wonders. We saw plenty of out-of-nowhere performances in 2019, and surely some of them will follow those seasons up with similar production in 2020 and beyond. But, just as surely, many of them won't.

We see it every year — in 2019 Brandon Nimmo, Jesus Aguilar, David Peralta, Trevor Bauer, Blake Snell, and Kyle Freeland all struggled to get even close to the numbers they put up in 2018, and all surely cost countless Fantasy owners dearly. Some of them have the injury excuse, but even for those players, the numbers weren't anywhere close to what they did the previous year.

Figuring out who took a real step forward and who was just a mirage is a key way to separate yourself for Fantasy; it allows you to avoid overpaying coming off a career year, while also finding those players who were just setting a new floor.

We talked about the biggest one-season wonders if the last 20 years on Tuesday's episode of the Fantasy Baseball Today podcast, and it was a helpful exercise for looking for candidates to avoid for 2020. It's always useful to look back at previous examples to see how things can go wrong. In some instances, there's no predicting it: You simply couldn't know Nimmo wouldn't stay healthy, or that Snell would deal with an elevated BABIP — he's actually a great candidate to bounce back.

However, some of the past candidates revealed plenty of warning signs: Flashes in the pan who couldn't sustain their breakout even for the whole season; players who didn't have the underlying numbers to support the breakout; and sometimes you will benefit from just betting against the outlier performance.

Let's take a look at some one-year wonder candidates from 2019 and whether they'll fall victim to it:

One-Year Wonders?
CHW Chi. White Sox • #10 • Age: 26
2019 Stats
AVG
0.315
HR
25
R
83
RBI
79
SB
10
SO
154
I actually believe in Moncada, personally, but you can't leave him off a list like this. Moncada had one of the highest batting averages on balls in play in MLB history, so there's regression coming no matter what. The question is how much, as well as whether he can improve enough to overcome it in other areas. I believe he can, even if he's more likely to hit .280 than .315 ever again. Moncada has one of the highest career BABIPs we've ever seen, and he hits the ball hard and runs fast, two key factors in sustaining that. The key might be sustaining his improvements against lefties, who he posted a .299/.345/.500 line against after struggling mightily in his first few seasons. I'm hopeful, because Moncada clearly benefited from a new more aggressive approach at the plate that allowed his considerable underlying gifts to shine.
ATL Atlanta • #12 • Age: 29
2019 Stats
AVG
0.265
HR
48
R
95
RBI
117
SB
3
SO
178
Soler looks like a pretty classic one-year wonder for me, because so much went right all at once for him, both in terms of staying healthy and reaching a new level of production. Soler has never lacked for power, but he led the AL in homers, thanks in large part to a step forward in strikeout rate; he especially improved after the All-Star break, when he hit 25 homers in just 71 games. Soler was having a solid season before then, but it was more like a standard Khris Davis season than what his final numbers ended up being. That would still have value, but taking him in the top-100 is making a big bet on at least some of the gains from the second half carrying. That's too much for me to ask, especially from a guy who had also never played more than 109 games in a season before 2019. If we're dinging Giancarlo Stanton for injuries, Soler should be right there with him.
TOR Toronto • #10 • Age: 31
2019 Stats
AVG
0.285
HR
33
R
123
RBI
92
SB
10
SO
102
I don't mind buying the breakout season when you don't have to actually pay for it, and that's the case with Semien, who is being drafted 83rd overall on average in 2020 after being a top-20 player last season. 2019 was surely an outlier for Semien, though he had hit nearly 30 homers once before. One thing you can say with certainty about Semien is he should be out there — he has played at least 155 games in four of five seasons — and will at least give you some homers and some steals. There's a floor here, where there isn't one for Soler. However, Semien also hit the ball harder more consistently in 2019 while building on the gains he made as a contact hitter in 2018. Add it all up, and I think Semien's a nice buy coming off a career year.
ATL Atlanta • #40 • Age: 24
2019 Stats
INN
174.2
W
13
K's
142
ERA
2.68
WHIP
1.11
In another era, the Fantasy community would've paid way too much for Soroka coming off a season that good as a 21-year-old. Now, he's being priced reasonably, thanks to a suite of advanced that peg Soroka as more like a mid-to-high 3.00s ERA pitcher than an ace. You probably won't get a big return on Soroka as the 25th pitcher off the board on average, and I like a few other potential one-year wonders going behind him a bit more — Frankie Montas, Sonny Gray, and Lance Lynn, primarily — but Soroka also isn't likely to hurt you. He's fine, even coming off a season he may never match.
OAK Oakland • #47 • Age: 28
2019 Stats
INN
96.0
W
9
K's
103
ERA
2.63
WHIP
1.11
Speaking of Montas, there's a lot to like in what he did in 2019, even though it was a career outlier. He has always had plenty of fastball velocity to spare, and there's not really any reason to think the PED suspension he took played a part in the breakout. No, this wasn't about some kind of velocity spike; Montas learned a new pitch, and it was a killer. Montas added a splitter to his repertoire and it immediately became his best pitch, inducing whiffs 40.3% of the time hitters swung at it. The addition of that splitter also helped the fastball and slider play up. We're only talking about 96 innings due to the suspension, but I'm willing to buy Montas, even with a somewhat elevated ADP of 110th overall.
CHW Chi. White Sox • #31 • Age: 32
2019 Stats
INN
85.0
S
25
K's
124
ERA
1.80
WHIP
0.96
Relievers tend to have one-year spikes more often than any other position for one key reason: They simply play less than anyone else. Soroka threw 174.2 innings in 2019, and there still wasn't enough time for his ERA to regress to where his underlying stats suggest it should've been. What do you think happens when you're dealing with samples in the 80-inning or lower range? Montas was hit with a significant suspension and still threw 11 more innings than Hendriks. Hendri0ks mostly has the underlying stats to support his breakout, though a 3.21 xFIP suggests he had some good luck on home runs that may not translate to 2020 as a fly-ball pitcher. Hendriks also had a velocity jump to help his stuff play up. There were real improvements here. However, you'll never go broke betting against breakout relievers, especially on a team like the Athletics, which hasn't had a pitcher record 20-plus saves in consecutive seasons since Grant Balfour in 2012 and 2013. Twice since they haven't even had one player record 20 saves. There's no security in this role.