When September began, it looked as if Carlos Gonzalez would enter free agency on the heels of a season-long slide.
The three-time All-Star, batting .239/.308/.356 at that juncture, had not pushed his 2017 OPS above .700, with the exception of three days in late May. With Gonzalez's 32nd birthday approaching in October, shortly before he would officially hit the open market, it seemed fair to wonder if these struggles were part of a steep and irreversible decline.
Then, a blistering September for the Wild Card-bound Rockies changed the dynamic.
Clubs in need of a left-handed corner outfield bat this winter now need to decide if Gonzalez's final month signaled a lasting resurgence, or was merely a small-sample blip in a troublesome season. It's a difficult question, in part because the degree of Gonzalez's late turnaround was unprecedented, at least by one measure.
Baseball-Reference.com's tOPS+ metric takes a player's OPS for a certain split (say, CarGo's 1.250 mark in September) and compares it to his overall OPS (.762). Gonzalez's resulting tOPS+ of 224 -- well above the break-even point of 100 -- is the highest produced by a player with at least 75 plate appearances in September/October and 300 for the season, going back to at least 1913. Just 11 others during that time reached the 200 tOPS+ plateau.
Gonzalez's September was spectacular. Of the nearly 300 batters who logged at least 50 plate appearances in that month, his park-adjusted weighted runs created plus (wRC+) of 204 trailed only Jose Ramirez, J.D. Martinez and Aaron Judge.

To be sure, there are mitigating factors beyond the small sample size. No player posted a higher BABIP in September than Gonzalez's .489, and he batted an absurd .513/.604/.897 in 48 plate appearances at Coors Field. Even during that red-hot month, he went just 5-for-27 (.185) with a .659 OPS in 28 plate appearances against left-handed pitchers, who have held him to a .635 mark since the beginning of 2014.
Those concerns aside, there is reason for clubs to take a bullish approach in their evaluations of Gonzalez.
First, his offensive outburst came after he fixed some bad habits related to his grip on the bat. He also began seeing a sleep specialist, who helped him get some much-needed rest. Beset by insomnia for much of the season, Gonzalez had been taking the field already sapped of energy.
"It's amazing what sleep can do for you," Gonzalez said prior to Colorado's NL Wild Card matchup against Arizona.
The well-rested version of Gonzalez put up September numbers that, while probably unsustainable, also were backed by impressive Statcast™ metrics. Of the 217 players who put at least 50 balls in play from Sept. 1 on, he ranked:
• 5th in average exit velocity (92.8 mph)
• 3rd in average exit velocity on liners/fly balls (99.8 mph)
• 3rd in hard-hit rate* (56.6 percent)
• 11th in barrel rate (15.1 percent)
• 19th in expected wOBA(.399)
*Exit velocity of 95+ mph
Only prolific sluggers Judge and Martinez notched a higher percentage of hard-hit balls over the season's final month. And since xwOBA attempts to strip out factors such as park effects and luck by focusing on a hitter's quality of contact -- plus actual strikeouts and walks -- the fact that Gonzalez's .399 September mark essentially matched Giancarlo Stanton's season-long xwOBA can be taken as a sign that Gonzalez earned a good deal of his success.

His damage included a pair of homers on Sept. 12 at Arizona, the first sailing a projected 427 feet and the second -- a 113.2-mph rocket -- soaring 478 feet to finish as the 11th-longest big fly in the Majors last season.
Four days later against the Padres, Gonzalez crushed a 426-foot drive into the second deck of right-field seats at Coors Field. And on Sept. 30, he ripped a 427-foot homer off Clayton Kershaw.
That solo shot, in Gonzalez's last regular-season game, provided an appropriate finale to an extraordinarily productive September. Whether his historic turnaround will be enough to override clubs' concerns amid a stalled free-agent market remains to be seen, but the outlook certainly looks rosier than it did during the dog days of August.