It's Hillis' second concussion in the past 12 months. He got one in the Giants' Dec. 15 home loss to Seattle and missed the following week's game in Detroit.
"It's tough," Hillis said of being put on IR. "You know, because you're right, I've been trying to get my career going again and it's just been really tough because I haven't had a lot of opportunities. But, I've been blessed to be on the team. It's crazy how one hit can take me out for the rest of the year. That's very depressing."
To replace Hillis on the active roster, the Giants signed linebacker Justin Anderson from their practice squad. Anderson can take Hillis' place on special teams and provide depth at linebacker, where the Giants are short this week because starting weakside linebacker Jacquian Williams will miss Sunday's game due to his own concussion.
Hillis will have another MRI on Monday.
"They told me because since this was my third concussion, they didn't want to take it too lightly," he said. "So, we are supposed to take another MRI on Monday to make sure that everything is still working fine because I'm still having symptoms right now."
What kind of symptoms?
"You know, the fogginess. Headaches come and go. Stuff like that," Hillis said.
Hillis is the second running back this week to be placed on season-ending injured reserve, following Michael Cox, who broke his leg in the Seattle game. The Giants signed running back Orleans Darkwa from the Miami Dolphins' practice squad for depth, and they also get starting running back Rashad Jennings back this week from the knee injury that sidelined him for the past four games.
The Giants now have 15 players on injured reserve, including guard Geoff Schwartz, who is eligible to return as long as they activate him no later than Monday.
ESPN's Josina Anderson contributed to this report.