And in 2013, Chao was singled out by DeMaurice Smith, the head of the players’ union, who called for the Chargers to fire him.

Chao’s fellowship with the American Association of Orthopedic Surgeons remains suspended, and he has not been certified with the American Board of Orthopedic Surgery since his 2014 probation.

Asked about these issues, Chao defended his credibility as a doctor, and a number of Chargers players have come to his defense over the years. As for his growing Twitter presence, he maintains that his long experience on the sideline should establish him as a credible commentator. And he has become more savvy about the “gotcha” culture of social media.

For instance, when he spots an injury, he now typically says, “By video, it appears to me. …”

“I’ve adapted,” Chao said. “It’s making it clear that I’m not reporting from a source. Believe it if you want, but it’s my impression.”

Chao also acknowledges to his followers that his opinions are not always perfect. But he also frequently cites his accuracy percentage, which he says is above 90 percent since he started posting in 2013, not long after he stepped away from his post with the Chargers, a position he had held since 1997.

It was then that he started a Twitter account under a pseudonym, @ProFootballDoc, to talk about the injuries he was observing. At first, he said, he wanted his account to remain anonymous, but he eventually attached his real name to the feed.

“I certainly didn’t have a thought-out plan,” he said. “I thought I’d try it for a few weeks. Very quickly, it caught on.”