Warren Moon once started a game at 44 years old — something Philip Rivers may also do come Sunday — and the Pro Football Hall of Famer is reacting to the shocking signing, and offering advice, ’cause playing quarterback is never easy, especially in your mid-40s!

“We’ve seen guys like Joe Flacco come back and do great coming out of retirement, but he wasn’t in retirement very long,” Moon told TMZ Sports this week. “In this situation, I’m a little more alarmed because he’s been away from the game for so long.”

Moon also wonders if Rivers, who may never have been Michael Vick, will still have the athletic ability to compete at the highest level.

“Does he still have those fast-twitch muscles that it takes to go out there and play this game, which has gotten probably a lot faster since he retired from the game five years ago? So those are the things that concern me as far as his reaction time and how he can get the ball out of his hand quickly enough.”

Moon knows about playing well into his 40s. In fact, if Rivers plays on Sunday against the Seahawks, he will be 44 years and 6 days old, just two days younger than Warren’s age for his last NFL start.

We asked the legendary QB what advice he had for Rivers.

“Rely on your running backs,” Moon said.

“They’ve got a great running back behind him that they can lean on, and their offensive line is going to have to really play well, really play physical to control the line of scrimmage and keep him out of long yardage situations.”

Warren continued … “Because I think that’s where he’s going to be hurt the most if he has to go with those, you know, 3rd and 8, 3rd & 9, 3rd & 10s, where he might have to hold the football a little bit longer than he wants to. And does he have the escape ability to move around in the pocket like he used to when he played, having been away from it this long?”

Ex-NBA player Jason Collins — the league’s first openly gay player — just opened up on his serious health battle … revealing he was diagnosed with Stage 4 glioblastoma, “one of the deadliest forms” of brain cancer.

Collins, who announced he had a brain tumor earlier this year, detailed his journey in an interview with ESPN‘s Ramona Shelburne … saying he first noticed symptoms back in August.

“Imagine a monster with tentacles spreading across the underside of my brain the width of a baseball,” Collins said … adding it’s impossible to fully resect the mass without coming out of the procedure “different.”

“My glio is extraordinary for all the wrong reasons, and is ‘wild type’ — it has all these mutations that make it even more deadly and difficult to treat.”

But Collins fought — starting with medication, then radiation and chemotherapy … and it worked. He said he slowly started to come out of his fog.

Even with all the treatment, Collins said the average prognosis is 11-14 months.

“If that’s all the time I have left, I’d rather spend it trying a course of treatment that might one day be a new standard of care for everyone.”

Tyler Robinson — the 22-year-old man who prosecutors say murdered Charlie Kirk — made his first in-person court appearance Thursday.

Robinson’s father, mother and brother were present as both sides were able to come to an agreement with regard to the degree of media access granted throughout the ongoing court proceedings.

Robinson previously appeared in court a few days after the shooting, via video call. Today, he sat in the Provo, Utah courtroom wearing street clothes … a privilege his lawyers fought for in a motion to ditch the jail garb.

As you know, Kirk was shot and killed on the Utah Valley University campus in Orem, UT in September … and, after an extensive manhunt, Robinson turned himself in to authorities.

Robinson allegedly confessed to the crime to his father, who then told a youth pastor who happens to work with the U.S. Marshals Service. Robinson ultimately surrendered at Utah’s Washington County Sheriff’s Office, around 10 PM on Sept. 11, the day after the shooting.

Utah prosecutors charged Robinson with aggravated murder, felony discharge of a firearm causing serious bodily injury, and obstruction of justice.