Month-to-month splits can sometimes be misleading. The baseball season is so long that mining one month’s worth of data to determine a trend can be a dangerous game. That being said, September (and the few days the regular season bleeds into October) can tell us more than other months.

September is a unique month for baseball in that rosters expand and the league sees a flood of replacement-level pitchers and hitters enter the league. As such, September can often be a month for fool’s gold.

Here are three players all going in the middle rounds of 2017 leagues whose strong Septembers might not necessarily portend that same level of greatness in 2017.

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Three September Surgers Who Could Struggle

Justin Upton (RotoBaller rank: 57; ADP in NFBC: 94)

Upton must have grown up a big fan of the Charlie Brown comics, and I’m guessing his favorite character was Lucy. He seems to lead us on every year, only to pull the football away once the actual season starts, leaving us on our backs, with a fifth-rounder producing like a 50th-rounder. In 2016, Upton spent the first three-quarters of the season making himself Public Enemy Number One for every fantasy owner who drafted him in 2016, before going beast mode in the final 37 games to totally redeem himself. Well at least partially redeem himself. As of August 20, Upton was slashing .228/.283/.374 and had just 13 HR. Then over the final month-plus, he tallied 18 HR and slashed .303/.395/.765 to bring his end-of-season totals to a respectable level (31 HR .246 BA) almost certainly in an attempt to draw us back in for 2017 fantasy baseball.

There was certainly some bad luck involved in Upton’s early-season struggles, as he posted a mere 3.7 percent HR/FB rate for the entire month of May and had a 10.2 percent HR/FB rate for the entire first half. Given Upton’s career 15.4 percent HR/FB rate, it's fair to say Upton started off a bit unlucky. But it was nothing compared to the insane good luck Upton got in September. For the final month of the season, Upton rocked a 35.1 percent HR/FB rate, well over twice his career rate. The jump was so extreme that his total season HR/FB rate (18.3 percent) ended up higher than his career rate and almost the highest rate of his career.

With Upton, a player who has never quite lived up to his prospect hype, it’s easy to imagine Upton thriving on some of the lower quality of pitching so prevalent after roster expansion. Of the 18 aforementioned HR, there are certainly quite a few off the dregs of the league. Here are some of the names Upton took deep in those final 37 games: Henry Owens (x2), Tyler Duffey, Ricky Nolasco, James Shields, Chris Beck, Alex Wimmers, Adam Plutko, Brandon Cuniff, Todd Sodburger and Aaron Blair. And I only made up one of those names. Upton got to Danny Duffy and Corey Kluber a couple times each, but he really picked on some of that lower-level talent. As someone who had Upton on his team throughout 2016, I’ll be avoiding him in 2017, even after his awesome end to 2016.

Yasmany Tomas (RotoBaller rank: 209; ADP in NFBC: 159)

The RotoBaller ranks don’t love Tomas, and I couldn’t agree more. On the surface, Tomas looks like a stud. He hit 31 HR in 2016, to go along with a .272 BA. He was one of only 16 players to reach those figures last season and he’s being drafted much later than most of those guys. On the other hand, Tomas has some of the worst plate discipline numbers in the league. For starters, his .313 OBP is truly heinous for a .272 hitter. Most leagues reward BA not OBP, but that dreadful OBP still really hurts his R totals, which came in at a pedestrian 72 in 2016.

It’s not as if Tomas is simply an aggressive hitter either. His 42.4 percent swing rate on pitches out of the strike zone was the fifth-highest in all of baseball in 2016. His contact rate on those swings? Just 57.6 percent, 25th-worst in MLB in 2016. That’s a killer combo, and it’s only a matter of time before opposing pitchers take true advantage of that lack of plate discipline. It’s not just his lack of plate discipline either, his swing has some serious holes in it as evidenced by his 16.3 percent swinging strike rate - second-highest among qualified hitters in 2016. That evidence alone should be enough to show that Tomas’ BA should drop into the .240-.250 range, but what about his power?

Tomas would still be serviceable in 2017 if he could hit .250 with 30 HR. However, Tomas relied on a fluky 25.0 percent HR/FB rate in 2016 that was powered by an impressive hard hit ball rate (41.0 percent) but was hindered by a poor fly ball rate (31.4 percent). One could see that and say “once Tomas starts to hit more fly balls, he’ll hit even more home runs,” or they could say “if Tomas doesn’t continue to hit the ball harder than Kris Bryant in 2017, his HR/FB rate is going to drop off quickly.” I’ll take the latter of the two statements, and I see Tomas coming down to the 25 HR range. When paired with a BA due for regression and extremely mediocre R and RBI totals, Tomas loses a lot of his shine.

Dan Straily (RotoBaller rank: 332; ADP in NFBC: 350)

Maybe you don’t need talking out on Straily. He’s only going to be a target in NL-only leagues and most of you will see his ERA-FIP (second-worst in the league at -1.11) and flee for the hills. But there are undoubtedly some of you who will see Straily’s final month (4-1, 3.13 ERA, 8.68 K/9) and start to talk yourself into the five-year veteran. You’ll think about how the move to Marlins Park is only going to help a career 46.8 percent fly ball pitcher, and you’ll think maybe he’s one of those guys who will be able to beat his FIP for five or six seasons and I can keep getting a discount while others write him off unnecessarily.

Don’t do that!

Straily deserves to be written off, even in NL-only leagues. He’s a pitcher who can barely top 90 mph with his fastball and had an xFIP over 5.00 last season. I know he managed to get 14 wins for a dreadful Reds team last year, but he did that with a surprising amount of run support and insanely lucky peripherals. His .239 BABIP against was second-lowest in the major leagues and his 81.2 percent left on base rate was sixth-highest. Don’t go banking on those two trends to repeat themselves in 2017, you’ll be very sorry if you do. Don’t overthink this one, just stay away.

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