His goal against Hibernian on Saturday was brazen, glorious, and devastating, but more importantly it was also timely. He had replaced the injured Kris Commons at Easter Road and now that player's role as the creative heart of Neil Lennon's side will be taken by Forrest against Ajax this evening. It will not faze him, but it is the final indication that he has fully recovered from the virus that led to him being hospitalised for five days.
Forrest lost a stone in weight during his mystery illness. That was startling to his team-mates and the Celtic coaching staff, not least because they had initially feared that the player might be suffering from meningitis. Forrest had returned to Glasgow following Scotland's game away to Macedonia last month with a back injury which ruled him out of Celtic's weekend fixture. He was still due to travel to Milan for the opening Champions League tie the following week, but ended up being admitted to hospital.
"When you hear something like that you think, 'wow'," said Lennon. "We'd seen him on the Friday, but we thought he'd be okay to travel. He scared us all really. We first thought it could have been meningitis, but thankfully it wasn't as serious as that. We haven't got to the bottom of it, though, but we always knew he was going to recover. The 10 days with the international break gave him the opportunity to get some conditioning work and get his weight back up. We wanted to start him last Saturday, but he was a bit apprehensive. He came on sooner than we expected, but he looked great and I was really pleased with his contribution."
Forrest is a player that Lennon concedes he has "to keep an eye on all the time". He does not need his behaviour to be checked by the manager, but his enthusiasm and willingness to play can occasionally cause him harm. Forrest has been susceptible to muscle injuries and other strains, but he has also established himself as a player capable of explosive bursts of influence and the ability to drive at defences with pace and trickery.
Lennon, having first worked with Forrest in the development squad at Celtic, has consistently promoted the player and his potential. With Commons missing through injury, and a conundrum to solve in central midfield with Scott Brown being suspended, Lennon will turn again to Forrest. His faith is based on the conviction that the attacker has all the attributes to succeed at this level, and that Celtic will need that combination of pace, dynamism, ambition and poise if they are to force Ajax on to the defensive this evening.
"He can handle it, he's good enough," Lennon said. "Away to Udinese, he was the best player on the pitch, he changed the game [away to Spartak] in Moscow [last season] when he came on. If he's fit and healthy, which we hope he is, he's good enough to give any team problems. It's just a shame Kris is injured because we haven't had them on the same pitch in a [full] Champions League game. They're two very important players, but it's great having James back."
As Lennon expects Forrest to rise to the occasion, so too is he certain that the rest of his players will cope with the pressure of a match that Celtic must not lose if they are to retain their hopes of qualifying from the group. Ajax are in the same position, though, and both sides will treat this game and their subsequent tie in Amsterdam, as matches in which they must gather points since Barcelona are generally out of reach.
Including qualifiers, this is Celtic's 21st game in the Champions League under Lennon, so most of the players ought to be experienced enough to handle the expectations. He acknowledges that the ratio of chances taken is not high enough, but bridles at the notion that his side lacks creativity. The issue for Celtic is that the team needs to be ruthless, and a home game comes with an obligation to rouse the crowd as much as subdue opponents by forcing them on to the defensive. That is not a style that Ajax normally adopt.
"I don't think it will faze the players," Lennon said. "This year, with the qualifiers on top of the group stages, there's a wealth of experience at this level. We might shade it in terms of experience compared to Ajax, and we're at home. This team has been in pressure games for the last two or three years and handled them quite well. The mood in the camp was quite light [yesterday], there was no apprehension. The excitement is starting to build again."
So, too, is the understanding that this will be a pivotal moment for Celtic's season and their ambitions. Lennon understands that after qualifying from the group stages last season, there is an expectation that Celtic will continue to progress. He raises the departures of Gary Hooper, Victor Wanyama and Kelvin Wilson as cause for some realism, but then, at the same time, he reveals the demands he places on himself.
Lennon is driven by the need to succeed, to keep moving forward. The challenge is to overcome a well-schooled, technically adroit Ajax side with an attacking mentality. There are issues to resolve, such as where to play Georgios Samaras and how to construct the centre of midfield, but mostly Celtic need to impose themselves on a game that demands an authoritative, commanding display.
"We are very wary of Ajax," Lennon said. "I don't think they're in the same class as Barca, not many teams are, but they can play and they have goals in them. Getting into the Champions League was massive, then you look at the group and think it's going to be difficult to emulate last year, but the group's far from away from us. These games [against Ajax] will be crucial."