Guru Saj is Phoebe's herbalist, portrayed by Kevin McDonald.
Ross visits Saj after discovering something weird-looking on his backside that Saj identifies as a "koondis." Saj claims that Ross needs to love the koondis in order to get rid of it, but ends up removing it himself after it gets caught on his wrist watch when he makes a hand gesture while he's examining it.
Joey and Chandler go to see Saj at the end of the episode after their pet duck gets a cough. Saj isn't incredibly helpful as he suggests they try get it to eat a bat.
Though he never appeared onscreen with her, Saj appears to be on reasonably good social terms with Phoebe, as she had apparently informed him of Ross' habitual clumsiness prior to when he actually met the latter, given his reaction upon entering the room and seeing Ross had accidentally ripped out one of his drawers, stating in an irritated but unsurprised tone of voice "You must be Ross." Or simply, the person who had let Ross in the room (like a receptionist or a secretary) informed Saj about him.