Israel international Biton picked up a suspected thigh tear during his nation's 2-1 friendly loss to Slovakia.
Scotland winger Forrest, meanwhile, was forced to pull out of Gordon Strachan's squad ahead of last week's friendly win in Poland with a groin injury.
And Lennon told the Celtic View: "Nir and James both went for groin scans on Monday and we think Nir is going to be a bit longer-term than Jamesy, and we will need to wait and see what the results of the scans bring back."
On a more positive note for Celtic fans, centre-back Steven Mouyokolo and Swedish defender Mikael Lustig are both nearing returns to first-team action.
Former Wolves man Mouyokolo had barely featured for the Glasgow giants before he was struck down with a ruptured Achilles tendon in September while Lustig has been out for four months with a hip injury.
Lennon believes both are well on the road to recovery and will provide a welcome boost as he begins his preparations for next season.
"Steven Mouyokolo is not far away and is out running now," said the Parkhead boss. "Mikeal Lustig will be another three or four weeks and it will be good to have both of them back as they have been two long-term absentees."
The Northern Irishman also lauded Mouyokolo for his spirit while battling back to fitness.
He said: "He has worked so hard. He's been terrific, his attitude has been fantastic and the medical team are delighted with the progress he's making. He's had a horrific time with injuries and this one was totally out of left-field.
"He was really disappointed, but the attitude and the character he has shown to come back has been fantastic. He is ahead of schedule and we are hoping he will get some game-time before the end of the season."