LeBron James needs to believe in himself a little more after he struggled in the Cleveland Cavaliers' game-three loss to the Boston Celtics, according to team-mate J.R. Smith.

James had one of his worst games as defending champions the Cavs were stunned 111-108 by the Celtics in the NBA Eastern Conference finals on Sunday.

The four-time MVP's run of games with at least 30 points was ended at eight as he had just 11 points, six rebounds and six assists at Quicken Loans Arena, with Cleveland's series lead cut to 2-1.

When asked about James' performance, Smith said after practice: "Play confident. That's my only thing. 

"People of his stature, well, there's only like three, but people like him, you gotta play confident the whole night and play aggressive. I mean, it's the Eastern Conference finals. 

"It's not enough, for him, for what he does, for what he brings. It's not enough. He knows that. We know that. Just expecting him to be better in game four."

But as for how James can get to where he needs to be, Smith said he is going to do his best.

"I mean, I never have that problem. I've been confident every time I've stepped on the court, whether I'm falling out of bounds or shooting a free throw, confidence is something I never lack," Smith said. 

"That's my job as his teammate and as his friend is making sure he stays confident in what he does, and you know, just trying to get him out of it."

James and the Cavs face the Celtics in game four on Tuesday.