Former MLB pitcher Brian Matusz -- best known for his years on the mound with the Baltimore Orioles -- died from an apparent overdose ... TMZ Sports has confirmed.
Our sources tell us Matusz was found dead at his Phoenix, Ariz. residence on Jan. 6 ... with a white substance around his mouth and drug paraphernalia -- including a straw, lighter and aluminum foil -- near his right hand.
We're told it was Matusz' mother, Elizabeth, who discovered him .