Paul Pierce is finally letting the world know how he suffered a finger injury that required a trip to the hospital last week ... revealing he busted up the digit during a workout mishap.
The NBA Hall of Famer explained the situation to Keyshawn Johnson and Skip Bayless on Monday's episode of "Undisputed," telling the guys it all happened as he was trying to do a plank with a weight on his back last Friday.
Pierce said at one point during the core exercise -- the 45-pounder that had been placed between his shoulder blades came dislodged ... and then fell right on his hand.
Pierce said it busted open his finger so badly, he could see the bone. But, he swore to the guys he never really winced -- despite how gruesome the ailment was.
"I drove myself to the hospital," he said. "Got the surgery. Went home and drunk a cup of coffee. That's it."
The 46-year-old Celtics legend then insisted he didn't even need to be sedated as docs put a pin in the finger to make sure it heals properly.
Pierce promised the injury won't keep him from the gym for long ... in fact, he told Johnson and Bayless he's got plans to get back in the gym later Monday!