Luck was showered in boos from Colts fans on Saturday who were pissed at the 29-year-old for retiring just 2 weeks before the season.

Some ex-NFL players and reporters have also trashed Luck -- essentially saying he's screwing over his teammates, coaches and fans.

But, Tom Brady ain't hearing that crap ... essentially defending Luck during his weekly appearance on "The Greg Hill Show" on WEEI.

"Everybody wishes they could be healthy all the time," Brady said ... "It is a contact sport and he’s certainly had his fair share of injuries, so guys retire at different times."

Don't worry, Tom ain't going anywhere ... the 42-year-old QB made it clear he wants to keep playing as long as he can -- and says his family, friends, teammates and trainer are behind him all the way.

"It feels great when you have that support because a football season is like a marathon. There’s moments where it is very challenging whether it’s physically, emotionally, or mentally -- you need somebody to help you push through the hard parts because it’s not all easy."

Brady added, "It’s a great challenge, but it is very rewarding when you meet the challenge, too."

What's interesting is that Tom's wife, Gisele, has spoken publicly about her concerns for Tom's long-term health ... saying in 2017 he has concussions all the time that "we don't talk about."

She also went on Ellen in late 2018 and suggested she tried talking Tom into retiring ... but "wasn't very successful."

Tom's not the only star QB defending Luck -- Troy Aikman came out swinging at Doug Gottlieb, who tweeted, "Retiring cause rehabbing is 'too hard' is the most millennial thing ever."

Aikman fired back -- "That's total bulls**t Doug. What qualified you to decide how someone should live their life?"