Warren Moon tells TMZ Sports he can't figure out how in the hell the Texans sent DeAndre Hopkins packing last month ... saying, "You just don't trade away your best players like that."
"I was really shocked," Moon said of the Cardinals-Texans swap. "Because you just don't trade one of the best receivers in football."
If you missed it ... Bill O'Brien committed an all-time blunder jettisoning Hopkins and a 4th-round pick to Arizona for David Johnson, a 2nd-rounder and future 4th a couple weeks ago.
The move was universally panned on social media ... and when we spoke with Moon on Wednesday -- he says he's with the majority of the football world on this one.
"A guy who's been a perennial All-Pro the last three or four years," Moon says. "Still productive. He still has two or three years left on his contract. So, I'm tryna figure out why?"
In fact, Warren says he believes the trade was so one-sided ... even O'Brien's own quarterback, superstar Deshaun Watson, has to be pissed over the move!
"I'm sure behind-the-scenes, he's very disappointed that he had a weapon like that taken away from him," Moon says.
As for O'Brien's future in Houston after the trade ... Moon says his seat is going to be hotter than ever -- and tells us if the coach doesn't win big next season, he could have his power stripped away by 2021.