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.”

“[Philip’s] been away for five years, and even though he’s been around the game, he’s coached the game, he probably still talks about it with his all-offensive coordinator, Shane Steichen, and all those guys, you just don’t know how his body is going to react to getting out there every day, the grind of practicing, how his body is going to feel after a week of practice.”

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?”

Good chance we find out on Sunday!