Harvey was placed on the 15-day disabled list on Wednesday after experiencing discomfort in his throwing shoulder. Subsequent testing revealed symptoms consistent with thoracic outlet syndrome, a condition that results from excess pressure on the nerves or blood vessels between the rib cage and collarbone that can cause pain in the shoulder and neck.
Boras told ESPN the thoracic outlet syndrome is the reason Harvey has underperformed this season.
In 17 starts this year, Harvey has pitched for a 4.86 ERA with a league-leading 10 losses and four wins.
An All Star in 2013, Harvey missed the 2014 season after undergoing Tommy John surgery. Last season, he returned to pitch 216 innings across the playoffs and regular season, ignoring the recommended innings limit of 180 for a pitcher returning from the reconstructive surgery.
The Mets (47–38) are currently three games behind the Nationals for second place in the NL East. They are hosting Washington for a four-game series in New York for their final series before the All-Star break.
Harvey was scheduled to pitch for the Mets on Saturday, but 26-year-old Logan Verrett will instead face off against Max Scherzer.