Thurston was helped from the field during the Cowboys’ 34-26 defeat at Townsville and while scans ruled out a break, he is thought to have suffered a ligament tear.
A Cowboys’ statement said: “The preliminary results are that Johnathan has a tear to the Medial Ligament in his right ankle. A Specialist will be consulted early next week.
“Johnathan is unlikely to play for the Cowboys for the rest of 2010, however the injury should not affect any representative duties that he may be selected for at the end of the Season.”
The Cowboys finals chances ended weeks ago but the news will be another blow for under-pressure coach Neil Henry.