The surgery was a success but he requires two months of rehabilitation, which means Johnstone will be unavailable for the remainder of the Super 14 season.

His place in the squad has been taken by Owen Franks, 20, a younger brother of Crusaders and Tasman prop Ben Franks.

Owen Franks made two first-class appearances for Canterbury last year.

Franks' callup represents one of three personnel changes the Crusaders have been forced to make.

The changes have come in the wider training group, with Canterbury under-21 winger James Paterson and Canterbury lock James Broadhurst taking the places of the injured Afeleki Pelenise and Jonathan Poff respectively.