Nick Reiner — who was arrested on suspicion of killing his parents Rob and Michele Reiner — will NOT appear in court today for the first time since their grisly murders … he doesn’t have medical clearance.

Prosecutors were expected to decide whether and how to officially charge Nick with killing his parents … but that will have to wait, because Nick’s defense attorney, Alan Jackson, says his client is not medically cleared to appear in court.

As we first reported … Nick is on suicide watch at Men’s Central Jail in L.A., where he’s being held without bail. Jackson said this morning he has not received any filing yet from prosecutors … and he doesn’t believe the court has received any filing either.

TMZ broke the story … sources told us Nick was acting very strange Saturday night at Conan O’Brien‘s Christmas party … not interacting with guests, and his attire was out of place … it was a pretty ritzy event, but Nick was rolling around in a sweatshirt and seemed high on something. We’re told Nick got into a loud argument with Rob at the party, and Rob left with Michele shortly after.

Nick lived at home with his parents, but he opted to check into a hotel at around 4 AM Sunday, using his credit card. The reservation was made for just a day, but Nick never formally checked out.

When the staff came into his room later Sunday morning, they found the shower “full of blood” and blood on the bed.

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Chrisean Rock might be reconsidering becoming a pro boxer, ’cause she just learned what it was like to get hit in the face by one of the world’s best fighters — when Ryan Garcia accidentally rocked her during a sparring session!!

Yes, the welterweight boxer sparred with Chrisean during his Kick stream on Monday … and his natural instincts kicked in at one point in the friendly session.

At first, Garcia (24-2) was letting Chrisean throw all her punches, but when he tossed a quick one back, it landed — leaving Chrisean in shock.

Garcia admitted to being nervous at first — mainly protecting his balls — but Chrisean managed to swing with confidence.

“That actually kinda hurt,” Garcia said, walking away in a little pain. “Now I know why people be trippin’ on it.”

They also jumped into a freezing pool to finish off the workout.

This collab comes days after Chrisean got into a huge brawl in Compton, ignited when a woman slapped her for grabbing her phone.

Bystanders claim Chrisean held her own against multiple people during the melee, which gave her the idea to take her street-fighting talents to the boxing ring and have Garcia be her coach.