An early-season back problem was compounded by a virus which led to him spending time in hospital while his teammates were taking on AC Milan in Italy. The winger returned off the bench against Barcelona and Motherwell earlier this month and is now targeting a return to the starting XI when Celtic face Hibernian at Easter Road on Sunday.
"It's just been bad luck getting not well and having a wee niggle here and there," said Forrest, who was left out of the Scotland squad to help him regain full fitness. "I just need a wee bit of luck now to get a run of games. We have a lot of big games left and once all the boys are back on Thursday and Friday we can look forward to the next few weeks.
"When you haven't been well, you don't want to come back and do too much, so it's been a good wee break and I have been able to do some hard work. I feel better in training and I just need to try and get back up to speed fitness-wise."
Lee Wallace, the Rangers full-back, was also forced out of the squad through injury, but his manager Ally McCoist has revealed that he will not be facing a lengthy lay-off. The defender even has an outside chance of facing Brechin City in Saturday's League One tie at Glebe Park.
"Lee had the scan and we got the results late last night," said McCoist. "We had a look at them and they're not too bad, thankfully. We're very, very hopeful he can start training again in a couple of days, although I'd have to say he'll be a doubt for the weekend. You never know though and he's a fit lad who'll give himself every chance to play.
"We'll see how things go. Lee will be disappointed to have had to withdraw from the Scotland squad because I know he enjoyed the action he got in Macedonia. He's got a taste of it and he wants more. It's a big disappointment for him to be out but I don't see it as being a total disaster as he's good enough to continue his form and keep his place in the group."
There was worse news for Wallace's fellow defenders, Ross Perry and Chris Hegarty, who are both struggling to recover from ankle injuries. Perry has not played yet this season while Hegarty has been out for two months. "Sadly both Ross and Chris have had setbacks and they've each been to see the specialist once again," admitted McCoist. "We're just monitoring them with our physios and the doctor and it's as much a mental thing now.
"The blows they've had will be of massive disappointment to them but they'll have to handle that and they've a terrific unit in here that will help them along."