Instead of being in Italy the 22-year-old was in hospital, floored by a suspected respiratory virus which kept him in for four days and will keep him out of football for up to another 10. He is already a doubt for the visit of Barcelona on October 1.
Forrest has not had his problems to seek. Even before succumbing to the illness he was struggling with an ongoing sciatic nerve problem which flared up again while he was in Macedonia with Scotland. "It is a race to have him back for the Barcelona game," said manager Neil Lennon. "We could have done with a fully fit James on a big pitch like the San Siro, given the way the game started to get stretched a little bit.
"Nothing seems to be simple with him. He missed a lot of games last season. Every injury seems to have a complication but younger players are prone to these types of things. I'm hoping once he gets past this it will be trauma injuries rather than the sciatic problem. The problem with that is an inflamed nerve. You have to do specific stretching like pilates and he has a strict regime which we have already seen the benefit of. But he was away with Scotland and then he had this virus which has restricted what he can do with us. Now he is out of hospital and back home with his mum and dad in Prestwick.
"The doctors think it was a deep lined respiratory virus but they are waiting on a few more tests coming back. Nothing has been confirmed yet. He was in hospital from Saturday until Wednesday but he is out now and feeling a bit better. He has a bit more energy about him. We don't know where it came from. He came back from Macedonia, he was in on the Friday and he couldn't play because of the sciatic problem - and I haven't seen him since."
Celtic will face the familiar challenge to lift themselves for a routine SPFL Premiership fixture this afternoon - at home against St Johnstone - after the exhilaration of playing against one of Europe's great names in the Champions League. Lennon does not intend to make many changes to the team which lost 2-0 to the Italians, although Teemu Pukki, Nir Biton and Derk Boerrigter - Wednesday night's substitutes - are all likely to feature. "They've not had game time or much of an opportunity, but they will come in and make us better. They are going to have to play well to get into the team and I want them to do that.
"The St Johnstone game gives me an opportunity to play one or two of them. We have a League Cup game on Tuesday [at home to Morton] where we might see a bit more of the new boys. I hope it won't be hard to lift them for St Johnstone. You never know until on the day but they are in good spirits and they can take a lot from the performance."
Virgil van Dijk and Efe Ambrose are likely to remain as the central defensive pair and the partnership has shown growing composure and potential in recent games. "I think van Dijk is going to be a top player, we are definitely better with him in the team. The idea was to pair him with Kelvin but Efe is really taking on the mantle and he's been brilliant since the start of the season."
It used to be the case that Ambrose and Victor Wanyama were inseparable around Parkhead. One would not be seen without the other at his shoulder. Ambrose laughed when that point was made to him yesterday. Wanyama's departure to Southampton meant Ambrose had lost a team-mate, but not a friend. "I always talk to him on the phone, or on Skype. He's always around.
"We normally speak once or even twice a week but I have to focus on my own football schedule. I'll call him or he will call me when we have some free time just to say "hi" and "how are you doing?" It's nice and who knows? Maybe we will meet again in future."
Wanyama had better get used to the idea that his big pal is now hanging around with someone else, to Celtic's benefit. Ambrose and van Dijk have formed a promising on-field relationship. "I think it's a good partnership. We understand each other, and the most important thing is we both speak English. That helped a lot. There's an understanding there but after a few more games it will be even better.
"We don't just depend on individual players. We are a team, that's the most important thing. Victor and Gary [Hooper] were part of the team last season and we had a lot of success but the ones who have come in can fill their shoes and maybe more success will come."