The Clippers said Johnson is out indefinitely, but a source told ESPN's Jeff Goodman that Johnson is expected to miss 4-6 weeks.
"That's tough for a rookie," Clippers coach Doc Rivers said, according to the team's website. "He's not going to miss the year or anything, but just missing two weeks, three weeks, four weeks, for a guy who's that young, that's a hard break for him. But it happens."
Johnson suffered the injury Thursday in the fourth quarter of the Clippers' preseason victory over the Toronto Raptors.
Johnson, a 6-foot-10 forward listed at 230 pounds, was selected 25th overall in the first round by the Clippers out of North Carolina.