Michael McDonald is a unique vocalist. He sounds different. The man made a career out of being the greatest back up singer of all time. He started out doing backing vocals for Steely Dan + writing songs for others, and rode that to success to fronting the Doobie Brothers and later a good solo career. I loved the Doobie Brothers as a toddler, pointing to the record player for more DB music. I still love McDonald's work. Heck, I imitate him on demand and plan on going as him next Halloween as an excuse to just dress '70s for the night. What makes his voice so different?
He has a great range but enunciates poorly. That is what a lot of folks say when they discuss his vocal stylings. I usually refer to my imitation as pretending to sing like I have a piece of chocolate located underneath my tongue, and I don't want it to fall out while singing. People mock his weird 'hurha' mumbling, but McDonald enunciates. People are wrong when they say he doesn't. His secret is he pushes the vowels in words out more and is sharp and concise with consonants. He must make the proper shape to his mouth for notes (as if one is holding a lightbulb bulb inward in one's mouth), but the tongue placement is key. He really uses his mouth to sing and not his throat. He is the anti-Bruce Springsteen.
A perfect example of his style hiding the lyrics or even just that he does pronounce all syllables is in "Real Love". Listen to "Real Love" at minute 1:28 to 1:33. Do you know what he sang? Here are the lyrics: "Days and nights like a wheel that turns". How do they sound: "dAAAys an nIIIItes like a whEEEEl thAAAAht tUUU-HUUrns". After you see the lyrics, you'll hear all of the syllables. He truly does pronounce everything, but his overemphasis on vowels is what makes him unique. I may have figured you out, but don't stop making smooth music Michael!