That's particularly a concern with 2015 Silver Slugger winner Gonzalez, who had an eye-popping .972 OPS at home in 2015, compared to a more pedestrian .758 mark on the road. That's basically the difference between Miguel Cabrera and Mark Trumbo, and deciding which hitter you think he is makes for a massive difference in his trade value. But while it's tempting to simply take a Colorado hitter's road stats and project them over a full season, it doesn't really work that way -- and that should give confidence to any team interested in acquiring them.
Let's explain why. While playing in Colorado obviously helps a player's home performance, evidence shows it may also hurt their road performance. For example, over the past 10 seasons, the Rockies scored 4,596 runs at home, unsurprisingly the most in baseball. But over the same period on the road, they scored 3,089 runs, the fewest in baseball, 151 runs behind 29th-place Houston. In order for both of those things to be true, either Coors Field would have to elevate baseball's worst offense to play like its best, or a middle-of-the-pack team would have had to receive positive effects at home and negative effects on the road -- which seems far more realistic. You can call that "the Coors Field effect."
Why? There's no shortage of theories, most regarding how difficult it can be for Colorado hitters to adjust to the different way pitches move away from Coors, an adjustment most other teams don't have to worry about for more than a short series. (There's also evidence that opposing pitchers throw more fastballs at Coors than they do on the road, as breaking pitches move less at altitude, which in turn makes Rockies' hitters appropriately aggressive at home and too aggressive on the road.)
Gonzalez, for example, actually hit the ball harder away from home in 2015, per Statcast™: 92.7 mph on the road, and just 89.0 mph at home. But he also struck out just 19 percent of the time at home, and nearly 25 percent of the time on the road, and that's a team-wide trend. The Rockies had an 18.5 percent strikeout rate at home in 2015, the 24th-highest mark in baseball, yet a 23.9 percent strikeout rate on the road, the second-highest mark.
But we can do better than theories and hypotheses. We can look at examples. Since 2002, when the Rockies installed a humidor, there have been 18 Rockies hitters with at least 650 plate appearances in Coors Field. Setting aside several current Rockies and those with limited or nonexistent post-Colorado careers, we're left with five good test cases: Matt Holliday, Dexter Fowler, Seth Smith, Chris Iannetta and Clint Barmes. (Let's acknowledge that we've introduced some amount of selection bias, because players who had careers totally collapse after leaving Coors Field aren't included, though notable names like Brad Hawpe, Ian Stewart and Garrett Atkins were already badly declining at the end of their Rockies tenures anyway. We shall proceed.)
From there, it's simply a matter of looking at the home/road splits they had during their Colorado careers, and comparing them to what they did after moving on. We're using raw OPS instead of the far superior wRC+ because OPS is not park-adjusted, a flaw that's very useful in this particular instance. If you compare their home OPS with the Rockies to what they did in their new home parks, there's a definite and expected decrease ...
... but that was partially offset by an increase in road OPS:
This is why you can't just take the road stats for a Colorado player and project them across an entire season -- because a Rockies hitter is likely to do better on the road once he's not calling Denver home, as nonsensical as that sounds. Put it this way: they'll lose their home-field advantage, but they won't be at as much of a disadvantage elsewhere.
Let's take one more approach, this time turning to OPS+, which accounts for park effects and is set so that 100 is "league average." How did this group do after leaving Colorado?
Every single one improved, except for Barmes, who basically hit at his established averages no matter where he was. While five players is admittedly not a huge sample, there's no evidence that a Rockies hitter who goes elsewhere and gets regular playing time is going to fall apart. Unless you think Gonzalez's entire career to this point is due entirely to Coors Field, which isn't supported by evidence, he'll be productive anywhere.
All of which is to say: No, Gonzalez probably wouldn't repeat his Coors Field performance if he were calling some other city home. But there is reason to believe that his road performance may improve, making the net result somewhere in between. For two years and under $40 million, that's a player any contender in need of a bat should want -- even at somewhere closer to sea level.
Mike Petriello is an analyst for MLB.com and the host of the Statcast podcast. He has previously written for ESPN Insider and FanGraphs. This story was not subject to the approval of Major League Baseball or its clubs.