Scans on Wednesday night confirmed Mansour had suffered an ACL injury.
"It's obviously shattering news for Josh," Kangaroos Coach Mal Meninga said. "He has been a popular member of the squad and was excellent for us against Scotland last Friday."
Mansour missed the start of last season after undergoing ankle and shoulder surgeries following the 2014 Four Nations, and received a major setback when he severely tore his medial cruciate ligament against the Titans in Round 7.
