Hawthorn midfielder Jonathan will miss up to a month of football with a hamstring injury.
Clarkson said O’Rourke, who did not play a game in the JLT Series, suffered a setback in his pre-season just this weekend.
“O’Rourke came back this weekend but he has hurt his hamstring and will miss another three to four weeks,” Clarkson said.
O’Rourke also battled hamstring issues last season, which saw him miss rounds six to eight.
He played a total of seven games last season, in his second year with the Hawks following two seasons at Greater Western Sydney.
Meanwhile, Clarkson confirmed Grant Birchall, who missed the end of the JLT Series with a calf complaint, will play in round one.
Hawthorn play Essendon at the MCG in the opening week of the season.