Johnston, 18, has an avulsion fracture of his fifth toe and will undergo surgery to reattach the tendon to his bone.
The injury is related to bone development and can occur among some children and adolescents when the bone and tendon don’t fuse correctly when growing.
The AIS/AFL Academy graduate from North Adelaide will be immobilised for the next six weeks and should resume running in about eight to ten weeks.
The injury is a setback for the number 12 draft pick who had been playing good football for the Swans Reserves in the early part of the season.
