"We really feel for Ethan, who has been in incredible form and is such a hardworking and well-respected member of this team," Robertson said.
"Our medical team will do what they do best and ensure he has everything he needs to be right for the 2023 season. In the meantime, he'll find other ways to contribute to our group, as Ethan always does."
Blackadder's injury will also be a big blow for the All Blacks, who start their season with a three-test home series against Ireland next month.
The versatile loose forward was a key figure in the All Blacks' 2021 squad, earning nine test caps.
Tom Christie, who replaced Blackadder against the Reds, will start in the No 7 jersey in the Crusaders' semifinal against the Chiefs on Friday night.