That's Tom Brady going to bat for his old pal, Bill O'Brien, after the Houston Texans fired the guy just 4 weeks into the NFL season -- and it's clear TB12 thinks Bill got a raw deal.

"That was really tough to see. I’ve known Billy for a long time and I think he’s a hell of a coach," Brady told Jim Gray on his Westwood One Sports radio show.

FYI, Tom and Bill worked together for 5 years in New England from 2007 to 2011. O'Brien left to take over the football program at Penn State before accepting the head coaching job with the Texans in 2014.

"I think he does a great job," Brady said of O'Brien.

"Four weeks ago, everyone was so hopeful about what the season could become. You hate to see things transpire as they do and to lose a coach 4 games into the season doesn’t make a lot of sense to me. It’s always tough when you see that happen, especially 4 games into the year."

Brady also said he believes O'Brien was well-liked by his players, despite some reports to the contrary.

"Those guys really seemed to love playing for him."

"I think he has great leadership ability," Brady said. "It’s a very difficult part of the profession and I thought he did a great job in Houston over the years."

Of course, a helluva lot of people in Houston would disagree ... O'Brien (who was also the GM) traded away superstars like DeAndre Hopkins and Jadeveon Clowney -- without much to show for it.