On a promotional stop in Auckland before the Parker fight, sponsored by Hydr8 Zero Explosion, Durban-based South African Botha said he respected the 21-year-old South Aucklander, who he said was a better fighter than New Zealand heavyweight champion Williams.
"He's a much better fighter than Sonny Bill which is why I've got to be in better shape. I don't underestimate Joseph.
"I know he's got a lot to win by fighting me so he'll make sure he's ready and the Buffalo will make sure he's ready."
Of Williams, who he had in serious trouble in Brisbane before the fight was stopped after 10 rounds rather than the advertised 12, Botha said it was "very unlikely" a re-match would happen because the Sydney Roosters player would have too much to lose.
The man who has fought, and lost to, some of the biggest names in the sport over the past 20 years, including Evander Holyfield, Mike Tyson, Lennox Lewis and Vladimir Kitchsko, said having the right mentality was the key to his durability.
"Boxing is not being tough. It's a skill, it's hitting someone without being hit. It's not being tough and saying 'I can take a punch'."
Duco Events promoter David Higgins said experience was why Botha was selected to fight Parker. "It's a huge step up for Joseph ..."