The 2-time NBA champ spelled out the scary details of the incident on his social media page Sunday ... saying he was riding his bike in his neighborhood when a car pulled up behind him.

"As I looked back behind me," Allen said, "the car was gone, but, the minute I turned my head back to what was in front of me, it was too late."

Allen says he crashed into a tree branch and was violently thrown from his ride.

"I landed on my face, shoulder and hip," Allen said.

The 45-year-old says he was able to get up after the crash and walk to his house ... but he claims that's when he lost his breath and nearly passed out.

"My wife immediately took me to the hospital to make sure that I didn’t have any internal bleeding," Allen said. "Had my vitals checked and a few X-rays taken and all was ok."

But, Ray did have some gnarly injuries ... Allen showed photos of bloody road rash on his leg as well as a big scrape on his shoulder.

Allen also showed a pic of his helmet, which clearly took a large brunt of the blow in the crash.

"Thank God for my helmet because as you can see in the picture my head hit the ground and I didn’t even realize it," Allen said.

"My helmet saved me from a far worse fate."

Allen says he was hesitant to share the incident but wanted it to be a reminder to everyone to wear helmets while riding.

Several NBA stars reached out to the 10-time All-Star expressing gratitude for his health ... with LeBron James writing on his social media, "Glad you’re good my brother!!!"