EAST RUTHERFORD, N.J. -- Giants linebacker Keith Rivers said Friday his hamstring feels better, but he isn't sure about his availability for next Sunday's game at Philadelphia.

Rivers re-aggravated his hamstring against Carolina after missing the previous week's win against Tampa Bay.

Rivers, along with other injured Giants, went to the Hospital for Special Surgery in Manhattan on Friday.

"Day by day," Rivers said. "Just continue to work at it, rehab and treat it."

The Giants acquired Rivers from Cincinnati for a fifth-round pick to help bolster the depth among their linebackers. He played well during the preseason and there were hopes that he would carry that over to the regular season, but his body hasn't been able to cooperate.

He started at outside linebacker in the opener against Dallas and had seven tackles, including one for a loss, but was hampered by a hamstring injury. He missed the following week against Tampa Bay and only registered one tackle against Carolina because the injury flared up.

Rivers wasn't sure when the injury occurred but knows he aggravated it while he was running. He did not play any defensive snaps Thursday and was only on the field for two special teams plays.

"It's been very frustrating," Rivers said. "Frustrating is putting it mildly. You work really hard to show off what you did in the offseason and to have a little nagging injury hamper everything is really frustrating."