The defending champions have lost three of their opening seven games and James made it obvious what area of the court has been most disappointing when quizzed by reporters.
"It's simple," the four-time MVP said. "These first seven games, we're playing like shit, defensively. It's that simple. We're not a sugarcoat team.
"We came in and got right down to it. We're terrible on defence, and we've got to change that."
The Heat will be looking for their fifth victory of the season when they host the Milwaukee Bucks on Tuesday.