Kisnorbo set to miss World Cup
The defender was carried off in the early stages of his side's 2-0 defeat to Millwall at Elland Road on Monday.
Grayson said the 28-year-old former Hearts and Leicester defender, who has been on the fringes of the Australia squad, now had little chance of featuring for Pim Verbeek's team in South Africa.
"Unfortunately for Paddy, he has got a severe Achilles injury which will rule him out of the rest of the season and probably the World Cup as well," Grayson said.
"He's been outstanding for us all season and I'm gutted for him because his dream was to play in the World Cup and it looks like it has been shattered."
Australia have been drawn in Group D with Germany, Serbia and Ghana for the June 11-July 11 finals in South Africa.