You can see in footage from the broadcast of the game, Danley immediately fell backward and laid motionless for several seconds.
Really scary moment.
Trainers rushed to tend to the 59-year-old ... and, fortunately, he was able to get up minutes later and leave the field under his own power.
The Giants said after the game that Danley was "doing well" ... though he was experiencing a "slight headache."
This is not the first time Danley -- who's had a history of concussions -- has had a scare behind home plate ... back in 2008, he was hit in the jaw by a fastball, and a year later, he was hit in the head by a broken bat.