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"His game is as good as it's ever been."

That's Brett Favre saying he hasn't seen ANY decline in Tom Brady at 42 years old ... telling TMZ Sports TB12's teammates deserve most of the blame for his down year.

"I saw a quarterback still able to do what he normally does but the cast around him was not up to par," Favre says ...

"How many times did we see Tom frustrated this year because guys were not on the same page?"

Tom posted a near career-low 88.0 QB rating this past season ... and his Pats didn't make a deep postseason run for the first time in YEARS.

But, Favre tells us a TB12 decline wasn't the reason for that at all ... saying he believes Tom still consistently showed the same tools that made him the game's G.O.A.T.

"I didn't see any decline in his arm strength," Favre says.

Still, Favre acknowledges Brady might be moving on from the Pats after this season ... and as someone who famously left Green Bay toward the end of his career, he spoke with us about what Tom's mindset could be heading into this offseason.