The 13-goal striker, who is poised to sign a new contract at The Den, suffered a muscle strain during the 2-1 defeat at Norwich on Tuesday night.
Morison will definitely miss Saturday's visit of Doncaster and has also been forced to pull out of the Wales squad to face the Republic of Ireland next week.
"Steve has picked up a bit of a strain," confirmed Lions manager Kenny Jackett.
"Thankfully, it's not too bad and he will be back with us in around a couple of weeks."
Source: PA
Source: PA