What the gap actually measures
ERA tells you what happened. Expected ERA tells you what should have happened. Statcast builds xERA from the quality of contact a pitcher actually allows, exit velocity, launch angle, and strikeouts and walks, then strips out the defense, the sequencing luck, and the bloopers that found gloves. When a pitcher runs an ERA a full run or more below his xERA, he is not beating the model. He is borrowing runs from the future, and the loan always comes due.
The hitter version is the wOBA vs xwOBA gap, and it works the same way. A hitter whose surface wOBA sits 50 or more points above his expected number is cashing in on batted ball luck, not contact quality.
The reason this works as a trade timing tool is simple: the market prices the surface. Your leaguemates see the ERA, the wins, the hitting streak. Statcast sees the contact. The window between those two realities is where value gets moved.
Case one: Michael McGreevy
McGreevy is the cleanest sell high pitcher in baseball right now, and it is not particularly close.
Through June 30, per FanGraphs data cited by Athlon Sports, the Cardinals right hander carried a 3.12 ERA against a 5.58 xERA. That is a gap of nearly two and a half runs. Baseball Savant tells the same story from the hitter side: opponents have a .296 wOBA against him, but a .360 expected wOBA. The contact is loud. The results just have not caught up.
Look under the hood and the support beams are missing. His Stuff+ sits at 81, bottom fifth among qualified starters. His swinging strike rate is 7.5 percent. Hitters have barreled him at a 9 percent clip with a hard hit rate over 40 percent. And the glue holding the ERA together is an 83.3 percent strand rate against a league average around 72. That is not a skill. That is a coin landing on heads for three months.