Bobby "The Brain" Heenan wasn't just a legendary wrestling manager -- he was also INCREDIBLE when it came to throwin' down in the squared circle ... so says Triple H

The WWE executive tells TMZ Sports most people don't know about Heenan's wrestling skills -- "He was one of the best in-ring performers there was. He just chose not to do it."

And why not? 

"He wanted to be a manager. That's what he wanted to do."

Heenan was obviously damn good at the managing thing -- rising to legendary status managing stars like Andre the Giant, Ravishing Rick Rude, Mr. Perfect and more in his storied career. 

Triple H also says one of his favorite Heenan moments was his 2004 WWE Hall of Fame induction speech -- which Bobby performed without a single scripted note. 

R.I.P.