MMA Fighting confirmed the news with UFC sources.
The 23-year-old Cantwell (7-3) captured the WEC light-heavyweight championship before the promotion melded its division with the UFC's. Though he's gone 1-2 in the UFC, he's fought competitive bouts even in losing decisions to Luiz Cane and Brian Stann.
Earlier this year, it was reported that Cantwell's career could be over after medical tests found potentially serious but never publicly disclosed issues that knocked him out of a scheduled UFC 108 bout.
Months later, though, it was determined he could continue his fight career, but just days from facing UFC newcomer Stanislav Nedkov, bad luck has apparently struck again.
With time running low before the scheduled weigh-ins, finding a replacement fighter to face Nedkov was an impossibility, and he has also been scratched from the event.