The announcement came only 30 minutes before the start of the Auto Club 400 at Auto Club Speedway.
Hamlin had qualified to start 13th in the 43-car field. But under NASCAR rules, Hornish will have to start at the rear of the field due to the driver change.
This is the second consecutive year that Hamlin, 33, has found misfortune in Fontana.
At last year's race, a crash with