Jonathan Eig is the author of Ali: 'A Life.'

President Trump made clear Friday that he doesn't especially mind if athletes suffer brain damage, just so long as the public is entertained.

"Today if you hit too hard — 15 yards!" Trump said at a rally in Alabama. "They're ruining the game."

Perhaps he thinks more brain damage will help keep athletes quiet.

The president's remark came days after the news that former football star Aaron Hernandez, who killed himself in April at age 27, showed signs of severe chronic traumatic encephalopathy, a neurodegenerative brain disease.

The report on Hernandez generated little shock, in part because we've come to accept the fact that head injuries are an inevitable result of playing football. Former quarterback Boomer Esiason said in his radio show this summer that he probably had CTE. He added: "All football players probably have it."

What if it's true? What if everyone who plays football is likely to develop CTE, and what if we can prove it? What if we developed a test to diagnose CTE in active athletes instead of waiting for autopsies, as is currently the case? Scientists at Boston University say a diagnostic test could be developed in five to 10 years, possibly through blood-based biomarkers or neuroimaging.

Would it make a difference? Or would we continue to disregard the damage done to young athletes?

Muhammad Ali's career shows why even better testing might not help prevent CTE in athletes.

In the 1970s, the term "CTE" had not been invented. Boxers who had suffered brain damage during their careers — and almost surely had CTE — were commonly described as "punch-drunk." There was little discussion of brain damage among athletes in other sports.

But Ali's case was unique. Because he was so well known and beloved — arguably the most famous man in the world — Ali was often asked whether he was concerned about brain damage. Early in his career, when he was young and fast, he said he wasn't worried. But as he aged and slowed, he took more punishment, and it was easy to see the effects.

Ferdie Pacheco, Ali's ringside doctor, told me he detected signs of brain injury in Ali as early as 1971, after Ali's epic fight with Joe Frazier.

Did Pacheco tell Ali he was risking further damage? "Yes, I told him," the doctor said, his voice rising in anger. "Every goddamn day I told him. . . . I couldn't stop him. I tried."

By the late 1970s, Ali's wife noticed that one of the boxer's thumbs trembled. Ali himself complained that he lacked the stamina for long runs and had difficulty sleeping.

Ali's parents, who encouraged their son to take up boxing at age 12, told reporters that Ali — then in his late 30s — shuffled when he walked and mumbled when he talked. They wanted their son to quit. But Ali kept boxing, unable to resist the money and public adoration.

Ali's success as a boxer stemmed from his ability to take punches and remain standing. But he paid for it. In his 15-round win over Earnie Shavers in 1977, Ali absorbed an astounding 266 punches. In a recently published study, scientists at Arizona State University found that Ali's speech rate (syllables per second) fell by 16 percent immediately after the Shavers bout.

He regained some of the speech as he recovered from the beating. But less than three months later, he went back into the ring to face Leon Spinks. He not only lost that fight, he also lost more of his speaking ability.

Afterward, a reporter asked if Ali worried about brain damage.

"No," he said slowly. "That happens to people who get hit too much."

A few years later, he wasn't so sure that he avoided damage.

"They say I have brain damage, can't talk no more," he said in a 1981 interview — 10 years after Pacheco's warning. He was approaching his 40th birthday and said he wanted four or five more fights before retiring. "What's wrong with me trying it?"

Many people believe that as awareness of CTE rises, it will cause a decline in the popularity of football, just as happened with boxing.

That could certainly occur, but there is a second question posed by Ali's career: If scientists can diagnose CTE in active athletes, what will athletes do? Athletes in every pro sport play with injuries, knowing — or at least suspecting — that they are doing permanent damage to their bodies.

Will football players want to get tested for CTE? Will they decide that living a long life with a healthy brain is more important than their careers? Or will they conclude that the joys and financial rewards of football outweigh the risk?

One hopes the answer would be the former. But if Ali's career provides a lesson, the latter seems more likely.