Benedict Kiwanuka, 32, and his brother were riding together in Indianapolis when Benedict hit a car and was thrown 100 feet off his bike.
A report by WRTV says he was taken to the hospital in critical condition.
Mathias, 27, riding alongside his brother, was not seriously hurt, according to reports.
The player's agent sent a text message to NJ.com that said, "his client 'said he is okay but his brother is not. ... He is worried about his brother.'"
The television report said, "Investigators said they were told by witnesses that the brothers were speeding prior to the crash."
A police spokesman told the Indianapolis Star , "Benedict was not wearing a helmet. ... A helmet was found at the scene, but police think it was buckled to the seat."
Television station WTHR said Mathias "did not hit the car and pulled over to help." The driver of the car had minor injuries.
Kiwanuka, drafted by the Giants in 2006, is an Indianapolis native.