The 20 year old suffered a reoccurrence of the injury last Saturday in the game against Derby County, he lasted just eight minutes.
The problem has been an ongoing one for the last four weeks and the results of the latest scan are not good.
"Josh looks a bad one to be fair," Rovers boss Michael Appleton told the club's official website.
"It is a grade two hamstring injury which isn't great so that could possibly be a 6-8 week job.
"Dickson Etuhu is probably about ten days away from training whilst Dunny has had a knock in training so he has missed a few days.
"Adam Henley is probably a month away because it was also a bad hamstring injury that he did."