The Wrestler star signed up to tell the true story of Britain's first openly gay rugby player and threw himself into researching the Welshman, even tearing his bicep by arm wrestling with a bunch of players on a boozy night out.
But he's now pulled out of the film, leaving movie chiefs to search for a replacement, according to Thomas.
He tells Britain's Sun on Sunday, "I was talking to Mickey yesterday and basically we're now talking to other big-name actors who could play the part instead.
"We had talked about using computer-generated special effects with Mickey but we don't think that will be as realistic, so it's time to find someone new."
Inception star Tom Hardy is reportedly in talks for the role.