MIAMI HEAT PLAYER CLAIMS THAT JASON TERRY'S TRASH TALK . . . HAS GOTTEN INTO LEBRON JAMES' HEAD!!!

People are wondering WHY LeBron James hasn't been playing as well as he's supposed to . . . well according to his teammate - it's because of Jason Terry's TRASH TALK!!!

According to the insider, Jason Terry's trash talk goes WAAAY overboard. The insider explained, "He tells LeBron that [his fiance Savannah] is f*cking other players. He talks about f*cking LeBron's mom . . . [Jason Terry] says whatever he has to to get under LeBron's skin."

And the TRASH TALK seems to work. According to LeBron's teammate, "LeBron may [even] believe some of that [stuff]. It's effecting his play."

Dang LeBron . . . you let a man TALKING SLICK TO YOU f*ck up your game. Somebody better get this dude to MAN THE F*CK UP!!!