After missing the previous two games the forward sustained a reoccurrence of the injury after just 8 minutes in last Saturday's 3-0 FA Cup win over Derby County.
It was hoped initially that the injury wouldn't be too serious, but it now seems that the Norwegian international could be out for up to two months which is a bitter blow for manager Michael Appleton.
"Josh looks a bad one to be fair," admitted the Rovers boss. "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."
Rovers are back in action this Saturday when Bristol City visit Ewood Park. Tickets will be on sale right up until kick-off priced from £15 for Adults, £10 for Seniors and £5 Juniors.
Book online today at www.onerovers.co.uk
and collect your tickets from the Blackburn End collection point on the day of the game.