An MRI exam confirmed Henry's injury. He is scheduled to have surgery Tuesday.
"Obviously, it brought practice straight down," Coach Byron Scott said. "To know the kid the way I know him and how hard he's worked, obviously we're all very hurt for Xavier."
Henry was injured during three-on-three drills, Scott said.
"He just made a move, like most guys do when they mess up their Achilles. Nobody hit him or anything like that," he said. "He went down and pretty much said to us that he heard a pop. We helped him in the locker room. Kobe [Bryant] and [Carlos] Boozer are in there with him, but obviously, he was devastated."
Over the off-season, Henry underwent surgery on his right knee and left wrist. Earlier this season, he flew to Germany for "Regenokine" treatment on his still-troublesome knee.
Through training camp he struggled with back spasms, never seeming to regain the form that allowed him to average 10 points a game last season as a pleasant surprise for the Lakers.
Through 13 appearances with the Lakers this season, Henry averaged 2.2 points a game while shooting just 23.1% from the field.
The Lakers may request a hardship exception from the NBA to add a 16th player.
If granted, the Lakers would need to get back down to the regular-season maximum of 15 once Kelly is able to return.
Henry can be still be traded (after Dec. 15) or waived outright but he'll earn his entire $1.1-million salary for the season.
The Lakers also are eligible to apply for a disabled player exception, worth half of Henry's salary -- but $541,000 may not be a large enough figure to acquire another player (through trade, waiver claim or free agency).
The Lakers are still waiting to hear from the NBA on their request for a $4.9-million disabled player exception for Nash.
The NBA quickly granted the team a $1.5-million disabled player exception for Randle, but that was a relatively simple case, with little for a league-appointed independent medical examiner to investigate.
In Nash's case, an investigator is believed to be pouring through years of medical records before making a determination. Nash, who broke his leg in just his second game of his three-year contract with the Lakers, never truly returned to form, with chronic nerve issues developing after the initial injury.
The NBA has to agree that Nash is out for the entire season to grant a disabled player exception, which can be used by the Lakers to acquire a player making up to $4.9 million, either by waiver claim or free agency. The team would also be able to use it to trade for a player making up to $5 million.
A disabled player exception does not enable a team to increase the roster to 16 players; anyone acquired would need to be on a one-year contract.
Mike Bresnahan contributed to this report.