Shepard had been cleared from the concussion protocol on Friday but after reporting to the team on Saturday night that he wasn't feeling well and still not feeling well on Sunday morning, he was placed back in the protocol.
"He practiced fully this week and was limited with no contact the two weeks before that," coach Pat Shurmur said in a statement. "He told (Senior VP medical services/head athletic trainer) Ronnie (Barnes) last (Saturday) evening he didn't feel well and was sent for examination and evaluation. When he arrived for work this morning, he still did not feel well. At that point, the decision was made to continue in the concussion protocol."
Shepard suffered his current concussion against the Minnesota Vikings on Oct. 6 and has missed the Giants' past three games.
This is his second concussion of the season. He suffered his first concussion after the Giants' Week 1 loss to the Cowboys, complaining of concussion symptoms after the game. He missed the Giants' Week 2 game against the Bills after being placed in the protocol but returned in Week 3 to face the Buccaneers after being cleared.
Shepard, 26, was the Giants' leading receiver before the injury, with 25 catches for 267 yards and a touchdown. He signed a four-year, $41 million extension with the Giants this offseason.
ESPN's Jordan Raanan contributed to this report.