The 24-year-old sustained a hamstring problem in the Old Firm defeat against Celtic on Sunday.
Naismith has been struggling with a hamstring injury in recent weeks.
“Steven’s problem has been a difficult one for everyone at the club to treat and diagnose,” Rangers boss Walter Smith told the club’s official website.
“There are times when he starts to feel better and it seems he is able to play but then it flares up again.
“Obviously we’ll have to give enough time this time to make sure he fully recovers from the problem.
“I would say it is more than likely he will miss the trip to Portugal now and that’s a setback with us.
“Without having spoken to the doctor or the physios about it, I’d imagine he will have to take a fair amount of time out before he can come back.”
