The former Los Angeles Laker shared the news Sunday morning ... posting a pic of his boy Zay laid up in a hospital gurney -- middle finger flipped up toward his pop.

MB runs through the timeline of the week ... explaining Zay was first hit by a car on Wednesday while jogging -- a scary enough moment many might assume actually put him in the hospital.

But, turns out Zay took the hit just fine ... playing in the Mamba League Invitational -- a high school tournament created in Kobe Bryant's honor -- Friday.

That night though, Barnes says Zay came to him and his mom in terrible stomach pain. The two rushed him to the hospital where Barnes says doctors performed emergency appendix surgery on what he adds one doc called "the largest appendix he's ever seen that hadn't ruptured."

Barnes says his kid's doing fine now ... and, Zay's posted today as well sharing a photo of himself leaving the hospital and saying he's been discharged.