In fact, both the Balls and the Pelicans say they have solid medical evidence proving Lonzo's ankle is fine.

The man behind the claim is Alan Foster, who was fired from BBB and later sued by Lonzo. In his suit, Ball claims Alan stole more than $2 million from him.

Foster denies the allegations -- but he's REALLY pissed ... and has launched a new YouTube show promising to "expose" the truth about the Ball family.

In a video published Thursday, Foster claims he knows a secret about Lonzo's health -- "Nobody knows that he has torn ligaments."

Foster says Lonzo met with a specialist in Ohio earlier this year (when he was still on the Lakers) and an MRI and X-rays confirmed ligament damage to his ankle.

"A torn ligament at the point of your ankle where your ligament is torn, it's not gonna heal on its own. He's applying too much pressure, he's cutting left and right. This is a very sensitive area that has to be strong," Foster says.

Foster claims Lonzo was all set to undergo surgery, but when the Lakers found out -- they "freaked out" and told him to cancel the procedure. He alleges Lakers honchos sent the team jet to immediately bring him back to L.A.

Foster claims the Lakers believed if Zo went under the knife, it would hurt his trade value -- "No team would have wanted Lonzo with an ankle injury, recovering from surgery."

Lonzo was eventually traded to the Pelicans -- but Foster says the 21-year-old never had the procedure to repair his injury, and is at a huge risk of causing even more damage.

"If Lonzo doesn't get surgery, he's gonna have another season with the New Orleans Pelicans and he's gonna get injured again."

We reached out to the Pelicans and a rep told us, "The team strongly debunks this and was very aware of Lonzo's injury history and status [before we traded for him]."