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." Play video content

"[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 bod

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