Former NFL star Alex Karras has been hospitalized due to kidney failure – a condition doctors say has left the 77-year-old former Detroit Lions defensive tackle with just days to live.

Tom McInerney, a Karras family friend, told the Detroit Free Press he became aware of his friend's deteriorating condition Monday morning in a phone call from Karras' wife, Susan Clark. McInerney told the newspaper Karras was in the hospital and has been given only a few days to live.

Karras, known to many football fans as “The Mad Duck,” played defensive tackle for the Lions from 1958 to 1970 and was part of the team's infamous "Fearsome Foursome" defensive unit of the early 1960s. After football, he turned to acting and starred most notably in the ABC sitcom “Webster” (1983-87), alongside his real-life wife Susan Clark, as the titular character's adoptive father.

In recent years, Karras played an intricate role in the concussion lawsuit against the NFL involving more than 3,000 former players. He has had several health setbacks including cancer and dementia, presumably as a result of his time in the NFL.

In a statement released on Monday, Lions president Tom Lewand said, "The entire Detroit Lions family is deeply saddened to learn of the news regarding the condition of one of our all-time greats, Alex Karras. Perhaps no player in Lions history attained as much success and notoriety for what he did after his playing days as did Alex.

"We know Alex first and foremost as one of the cornerstones to our Fearsome Foursome defensive line of the 1960s and also as one of the greatest defensive linemen to ever play in the NFL. Many others across the country came to know Alex as an accomplished actor and as an announcer during the early years of Monday Night Football.

"We join his legions of fans from both sports and entertainment in prayer and support for Alex, his wife Susan, and his entire family during this most difficult time."

Drafted 10th overall out Iowa by Detroit in 1958, Karras was a key player on the Lions defense.

Karras’ wife said he suffered from dementia symptoms for more than 12 years and the period leading up to his current condition saw him unable to get behind the wheel of a car, the Washington Post reported.

“This physical beating that he took as a football player has impacted his life, and therefore it has impacted his family life,” Clark said earlier this year. “He is interested in making the game of football safer and hoping that other families of retired players will have a healthier and happier retirement.”

The NFL has said it did not intentionally seek to mislead players and has taken action to better protect players and to advance the science of concussion management and treatment.