The fan had to be carried from the lower section of seats just beyond the visiting dugout down the third base line after apparently being struck by a liner off the bat of Yankees third baseman Todd Frazier.
While it is tough to see exactly what happened, fans began immediately waving for medical attention.
The girl is being treated at a local hospital, according to the Yankees.
The game was stopped for several minutes as the child was taken out of the seating area. The federal Health Insurance Portability and Accountability Act, known as HIPAA, prevents the team from giving more information. The team was declining further comment. Frazier knelt on one knee and bowed his head as the scene played out in the hushed ballpark.
According to St. Paul Pioneer Press reporter Mike Berardino, the child appeared responsive as they were carried out of the seating area.
Jose Mourinho mad about Sergej Milinkovic-Savic: Lazio could sanction £62m deal
The Serbia Under-21 global has since become a key player for Lazio , earning links to a number of Premier League clubs in the last couple of years.
It's not the first time this season a spectator has been injured at Yankee Stadium this season.
The age of the person hit by the foul ball and the extent of her injuries is not yet clear, as the game's broadcast chose not to show the fan on camera. In May, a young boy was left bloodied after being struck by a broken bat at Yankee Stadium.
In July, a man was hit by a foul ball that left the bat at 105 miles per hour, according to the New York Daily News.
The Mets extended the netting down the first and third base lines following the All-Star break this season.
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