Rhinos were thrashed 56-12, their worst defeat for four years and lost Tom Briscoe, Carl Ablett and Ashton Golding with ankle injuries in the first half.
It was a nightmare scenario for Rhinos, who face North Queensland Cowboys in the World Club Challenge next weekend.
Kallum Watkins scored for Leeds on the final play of the first half to make it 28-6 and Rob Burrow crossed late in the second, both converted by Jordan Lilley.
But Leeds were out-played in every department and were 16-0 down before their first injury.
Corey Thompson scored a hat-trick for Widnes, who won both halves 28-6. Other try scorers were Stefan Marsh (2), Charly Runciman , Lloyd White, Joe Mellor, Setaimara Sa and Connor Farrell.
Rhys Hanbury landed eight conversions.
An outstanding performance by Man of Steel Zak Hardaker was Rhinos’ only consolation.
Check back later for a full report.