MIAMI -- The Miami Heat's sluggish play isn't sitting well with LeBron James, who vented his frustration after Monday's practice amid the team's 4-3 start to the season.

"It's simple," James said as the Heat prepared for Tuesday's home game against the Milwaukee Bucks. "These first seven games, we're playing like s--- defensively. It's that simple. We're not a sugarcoat team. We came in and got right down to it. We're terrible on defense, and we've got to change that."

James' comments came on the heels of the Heat's 111-110 loss Saturday to the Boston Celtics, who won on a buzzer-beating 3-pointer by Jeff Green after overcoming a four-point deficit in the final 3.6 seconds. The shot from Green capped a relentless performance by the Celtics, who took advantage of Miami's lethargic defensive effort throughout the game.

It's already developing into a disturbing trend for the Heat, who are ranked first in the league in offensive efficiency but just 25th among the NBA's 30 teams in defensive efficiency. Miami also ranks among the bottom third in the league in points allowed, yielding 100.9 per game.

The Heat opened Tuesday's practice with some harsh words from coach Erik Spoelstra and then spent the next two hours working to correct defensive concerns. After the workout, James twice used expletives during his session with reporters when asked the source of the team's poor play.

The early-season problems aren't new for the Heat, who were ranked 26th in points allowed through the end of November last season. That slow start eventually gave way to a 27-game winning streak and culminated in a second straight championship.