The late 70's brought some changes to the Doobie Brothers, particularly, the departure of vocalist-guitarist Tom Johnston, and the emergence of keyboard player Michael McDonald as the new lead singer. McDonald and guitarist Jeff "Skunk" Baxter were also studio players with Steely Dan around this time. "What a Fool Believes" turned into a big hit for the Doobies in 1979, moving them closer to the adult contemporary & pop charts and away from album-oriented rock (AOR).
After the Doobies initially called it a day three years later, McDonald released his first solo album, and now is more of a Motown cover specialist, or so it would seem. 36 years later, though, "Fool" still resonates.