The 21-year-old will miss his country's World Cup double-header against Wales and Serbia after he missed the Bhoys' 4-3 win over Aberdeen on Saturday due to a recurrent problem
The 21-year-old has suffered from an injury-stricken season and missed their dramatic 4-3 win over Aberdeen on Saturday due to the ongoing problem.
Scotland currently sit bottom of their qualification group and, with their double-header against Wales and Serbia imminent, Celtic manager Neil Lennon has confirmed that Forrest will not be able to feature.
"He is going to see a specialist on Tuesday, he may need an injection in his back which might keep him out for another couple of weeks," Lennon told reporters. "He will be out of the Scotland squad."
The Hoops boss admitted that Celtic would miss the young midfielder and explained that his still-developing body was the reason behind his nagging injury problems.
""He is a huge loss," Lennon admitted. "I think he is a class player and he has shown that but it has been stop-start for him.
"It is a concern for him, I don't think he is fully developed yet. Once he fully matures physically, you are hoping these things settle down."