Very sad....IIRC he and MacArthur Lane might have been the first to both have 1000 yards (or something like that) in the same season AND in fewer games.  Regardless, he was a terrific running back.

Packers Hall of Famer John Brockington passes away
In his first three seasons, Brockington rushed for 3,276 yards and averaged 4.3 per carry
Mar 31, 2023 at 04:23 PM
Green Bay Press-Gazette
Former Packers Hall of Fame fullback John Brockington
Green Bay Packers Hall of Fame fullback John Brockington, the first NFL player to rush for 1,000 yards or more in his first three seasons, passed away Friday. He was 74.
"The Packers family was saddened to hear about the passing of John," said Packers President/CEO Mark Murphy. "One of the great runners of his era, John was an exciting player to watch with his powerful running style. Fans enthusiastically welcomed John back to Lambeau Field over the years, fondly remembering the 1972 division championship as well as the bright spots he provided in the less-successful seasons.
"I enjoyed getting to know John during his return visits to Green Bay and greatly respected his work in support of organ donation through the John Brockington Foundation. He leaves a wonderful legacy.
"We extend our condolences to his wife, Diane, and his family and friends."
Playing in the era of 14-game schedules, Brockington rushed for 3,276 yards and averaged 4.3 per carry in his first three seasons.
His first year, 1971, saw him set the NFL rushing record for rookies with 1,105 yards, while averaging 5.1 per carry, and he was the only rookie named Associated Press All-Pro. He also was named the NFL Offensive Rookie of the Year by the AP, and NFC Rookie of the Year by both the United Press International and Newspaper Enterprise Association.
The Packers traded up and selected Brockington out of Ohio State with the ninth overall pick in the 1971 draft. In all, he played six seasons and one game for the Packers, finishing with 5,024 yards on 1,293 rushing attempts, a 3.9 average, and 138 pass receptions for 1,075 yards, a 7.8 average.
Brockington was born Sept. 7, 1948, in Brooklyn, N.Y.

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It is sad news. John was one of the few rays of sunshine in the depressing 70s. I would marvel at what he could do. 

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The pleasure derived from watching John run the football was truly visceral in nature. The man would punish his would-be tacklers, and I truly mean the plural, not the singular. If you ever watch some of his tape, note how so many of the ends of his plays kind of resemble a multi-car collision with the wreckage of bodies spread down the field. He could absolutely stone cold truck defenders. What
an awesome memory. Rest in peace, Mister Brockington, sir