Here's a thought experiment for you. Let's say a friend who has never really paid attention to fantasy baseball before comes up to you in the middle of June and sees you're setting your lineup, and he then asks you to explain the concept of the game. You happily do so, using this quick and easy-to-understand explanation: For hitters, batting average, home runs and RBIs are good, while strikeouts are bad.
One month later, you ask your friend if he's been watching. Indeed, he has! You ask who he would pick with his No. 1 overall pick if drafting a new team today. Would it be Mike Trout? He starts laughing at you, "Why would I pick him? He's horrible." And, based on the evidence, he'd be right.
From June 19 through July 21, Trout hit just .224, with 2 home runs, 4 RBIs and 26 strikeouts. Compare that with the most amazing player in fantasy baseball, Omar Narvaez, who hit .431 with 2 homers, 10 RBIs and just 8 strikeouts during that same stretch of the season. Which stats would you rather have? It's a no-brainer.
Obviously, I've cherry-picked a really sad stretch of Trout's 2018 season, and done so in hindsight. Given the choice, there's no way I would have started Narvaez over Trout in my utility spot -- nor would I have gone with Jesse Winker (.423, 4 HR, 24 RBI, 13 K) or Steve Pearce (.391, 2 HR, 9 RBI, 8 K), even though all three bats were far more valuable to a fantasy team over this small sample size.
However, the reason I bring this up is to point out that, during any three-week stretch of games, the possibility exists to see one of your star players go totally cold. The possibility exists for a not-so-impressive role player to suddenly get hot and surge to the top of the list of recent scoring leaders. So, if all that's left in your fantasy league's regular season is a handful of games, in a way all bets are off.
Essentially, unless you're in a dynasty league, we're all playing a form of DFS at this point of the season. Since August started, a lineup of David Peralta, Adam Frazier, Mallex Smith, Austin Jackson and Renato Nunez has vastly outperformed a lineup of Shin-Soo Choo, Charlie Blackmon, Alex Bregman, Nicholas Castellanos and Trea Turner.
Now, I'm not saying you want to bench Mookie Betts for Aledmys Diaz, but at the same time if you've got a struggling lineup mainstay, what's the harm in sitting him in a tough matchup, grabbing streaking "nobodies" off the wire and rolling the dice for a few days? Phillip Ervin, Franmil Reyes, Jeff McNeil and Isiah Kiner-Falefa may not be household names, but if guys like this end up giving you more fantasy points from here to the end of the fantasy regular season than Mitch Haniger, Eddie Rosario or Ozzie Albies, then why not roll the dice?