Sciatica is pain or numbness that follows the path of the sciatic nerve from the lower back down the leg.
The Lakers revealed the diagnosis of right-side sciatica after the team's practice, where the 40-year-old James again was not a full participant.
He missed the Lakers' first two pre-season games with what Lakers coach JJ Redick called nerve irritation in his glute.
The Lakers said James would be reevaluated in about three to four weeks and further updates would be provided at that time.
Redick said Thursday that James and Luka Doncic were limited to "modified, mostly individual work" at practice.
ESPN had reported on Tuesday that James would likely miss all of the pre-season but had hoped to be ready for opening night.
The four-time NBA champion and all-time points scorer told reporters on media day last month that he was "excited" by the prospect of becoming the first player in NBA history to play a 23rd season.
"Whatever the journey lays out this year, I'm super-invested because I don't know when the end is. It's a lot sooner than later," he said.