Eli Manning has a stress reaction in his foot. That's not the same thing as a stress fracture, but it increases the likelihood that Manning could get a stress fracture, which would end his season.
This stress reaction in the result of Manning compensating for the plantar fasciitis he has battled for much of this season.
Before any stress fracture develops, the Giants are aggressively treating the stress reaction, trying to get Manning to stay off it as much as possible, having him wear extra supportive shoes and using a bone stimulator on the injured area.
For now, the stress reaction is not enough to sideline Manning. But Manning has been spotted favoring the foot, which he was doing as recently as Thursday night, when he was seen limping at times.
So yeah, not good.