Peacock, who was shortlisted for the Man of Steel award following an impressive campaign with Leeds Rhinos in the Super League, called time on his international career in June 2012.
"For me he's been the best forward in the competition this year, but obviously he made a decision and he's stood by it," BBC Sport quotes Sinfield as saying.
"He was weakening at one stage but he's a steely character. The last month or six weeks have been really tough on him away from the game and he feels it's the right thing for him to do."
The England squad for the tournament will assemble in South Africa this week.