Wei-Yin Chen is the weakest link in the Marlins starting rotation. He’s also their ace. It just depends which city they’re playing in.

With most of the 2018 season complete, Chen owns the most extreme home/road splits that have ever been recorded for a major leaguer with his kind of workload. Including Monday night’s gem, he has started nine games apiece at Marlins Park and away from it. These are his results:

Home: 1.94 ERA, 51.0 IP, 3 HR, 18 BB, 43 K

Road: 10.27 ERA, 37.2 IP, 11 HR, 18 BB, 23 K

The Fish are not competing for a playoff spot, but manager Don Mattingly still wants to put his players in position for individual success. Mattingly was so spooked by these splits, he used a postponed game last week as an opportunity to realign the Marlins rotation. He moved Dan Straily into Chen’s scheduled start on Sunday and pushed Chen back a day to kick off the homestand. Both veterans were successful.

Combined over the previous six seasons, the Taiwanese left-hander performed at a nearly identical level during his home and road appearances. He actually demonstrated reverse splits in 2016-2017 after signing in Miami as a free agent. The inconsistency of his current campaign came out of nowhere.

These comments by Chen to Clark Spencer of the Miami Herald are illuminating:

Chen and pitching coach Juan Nieves have gone over his game tapes, looking for any differences. So far they have found none. “Me and Juan, we’ve been trying to figure out why i have such a big (home/road) split,” Chen said. “I feel like I pitch the same way on the road as at home and he does to. He feels like I’m the same guy.”

Indeed, his stuff is the same regardless of venue.

Since being activated from the disabled list on April 28, Chen’s average fastball velocity has steadily crept up from 90 to 92 miles per hour. That seems to be indicative of building up arm strength. (He suffered a partially torn ulnar collateral ligament last season.) The spin rate on his pitches can fluctuate, but there’s not much correlation between that and his success. For example, Chen was throwing the slider—his primary breaking ball—at 2,079 rpm on July 2, then perked up to a season-high 2,321 rpm in his next outing. The July 2 game (at home) was among his best of 2018; the latter game (on the road) turned into a blowout loss.

The type of competition isn’t responsible for the splits. Chen excels against left-handed batters, so you’d think it would help that he’s been seeing them more frequently on the road (21.7 percent of plate appearances) than at home (19.1 percent). Nope—road lefties are owning him with a .308/.325/.526 batting line while home lefties look helpless at .105/.128/.211.

The quality of competition isn’t to blame, either. The 33-year-old has faced the Braves, Nationals, Cardinals and Rockies as both a host and visitor.

It gets weirder! Chen pounds the strike zone just as often on the road as at home. However, only 29.7 percent of road opponents who fall behind 0-2 in the count against him eventually strike out. That rate spikes to 44.7 percent under the same circumstances in Miami.

Why does Chen produce at an $80 million level at home, but get crushed like an 80-year-old man on the road?

There has been rotten luck involved. The discrepancy between his .230 home/.341 road batting average on balls in play comes down to the execution of defenders behind him. The bullpen also contributes to this. Marlins relievers have inherited eight baserunners from Chen in home starts and stranded seven of them, while being far less efficient otherwise. That’s reflected in his earned run average.

Meanwhile, Chen earns a big chunk of his awful road results. He’s nearly half as likely to allow a home run (6.0 HR%) as he is to pick up a strikeout (12.5 K%). Hence the 7.16 FIP, worst among all MLB pitchers (min. 30 IP on the road).

What’s the No. 1 issue behind all of this?

Location, location, location

Courtesy of Statcast, Edge % is self-explanatory: it tracks the percentage of pitches that cross the plate along the edges of the strike zone. High, low, inside, outside, as long as it’s on a border. With below-average velocity and ordinary break of his pitches, Chen must live on the edges to get into advantageous counts and induce weak contact. His magic number is 30.

Chen generally maintains a 30 Edge % or better this season. The exceptions have been May 4, May 9, May 31, June 6, June 22 and July 30. Guess what each of those dates have in common? Yup, road starts.

Pictured below are the pitch locations from Chen’s matchups with the Rockies on April 28 and June 22. He threw more strikes in Colorado...but not where he wanted them:

My first instinct was to tease Mattingly and the Marlins for buying into the significance of these home/road splits.

After further investigation, though, we find that Chen’s command continues to fail him outside South Florida. It was the culprit behind his first road start of the season, his most recent stinker in Atlanta and several ones in between. Sure, Coors Field is a pitcher’s nightmare, but how do you excuse the similar struggles at Wrigley Feld and Petco Park?

It’s a mystery that perhaps only Chen can solve himself.