Best, who joined from Newcastle for £3million this month, has been ruled out for six months with a cruciate ligament injury after being stretchered off in the 2-1 friendly win over AEK Athens on Sunday.
A tentative approach for Huddersfield's Jordan Rhodes was rebuffed by the West Yorkshire club and Rovers now face a race against time to strengthen their forward line ahead of the new campaign.
"What we have to do now is find a good replacement and also supplement that," Blackburn's global adviser Shebby Singh told BBC Radio Lancashire.
"When I heard the news it was difficult for me not to cry. Life has to go on, but right now it hurts very deeply.
"We were always going to look for someone else to deputise for Leon. Now we've got to look for someone who is going to play and another player who is going to deputise."