UnitedHealth is reeling from a nearly 20% stock rout after warning investors about its first revenue decline in decades

UnitedHealth is reeling from a roughly 20% stock rout after warning investors it expects its first annual revenue decline in more than three decades, a stunning reversal for a company long seen as one of Wall Street’s most reliable growth engines. The selloff wiped tens of billions of dollars off the health-care giant’s market value and sent shockwaves through the broader managed-care sector as investors reassessed the risks in the once‑high‑flying Medicare Advantage business.

UnitedHealth’s...