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 .