Waratahs centre Rob Horne has been ruled out of the opening rounds of this season's Super Rugby campaign with a hamstring injury.

Waratahs coach Michael Cheika has confirmed that injury prone Horne will be out of Super Rugby for at least one month.

"Definitely Horne's out... four to six weeks I'd say," Cheika told AAP.

Cheika also said that Wallabies winger Drew Mitchell is in doubt for Thursday's Super Rugby trial match in Sydney against the Crusaders due to a calf injury.

The Waratahs lost Berrick Barnes for the opening rounds and Bernard Foley has an ankle injury so Cheika said that he will start 22-year-old Southern Districts five-eighth Ben Volavola at 10 for the Crusaders trial.