His goal in the 4-0 win over Motherwell marked the then-18-year-old as someone destined to zoom to the top.
Forrest's inclusion in the game at Parkhead was a statement from interim manager, Neil Lennon, that things were going to change around the club which had, under Tony Mowbray, spent millions dragging in dozens of players from around the world.
They had been of mixed abilities, but united only to combine into a side which failed to fire in any competition - or in the hearts of the rapidly-disconnecting fans.
The arrival on the scene of Forrest marked a step change in the way things would be done under rookie Lennon, if he was given the job full-time.
Sure, the new manager would still look to every market available for players of the quality he needed to make Celtic winners again.
But, with financial restraints as they were - and with the demise of Rangers only going to make things even tighter - bringing through players from the ranks would be a huge bonus if not a necessity.
It would not only save the club money it was having to work hard to earn, but also provide the boost to the support which only cheering on a home-grown Bhoy can produce.
Now Liam Henderson, Eoghan McConnell and Darnell Fisher are following in the footsteps of Forrest in making their mark in the first team.
Ironically, as the Hoops prepare to face Motherwell again, the precocious winger finds himself on the outside looking in, the result of yet another injury.
The joy at winning three championships since breaking into the side has been tempered for Forrest by the pain of having to miss so many matches through a litany of problems.
His well-documented back nerve issues and hip and hamstring injuries have made his surge to the top of the game more of a stop-start process than anyone - least of all the player himself - could have predicted, nor wanted.
Yet, for all the reputation he now has as someone who can star one week, but be on the treatment table the next, the reality is that Forrest has already played for the Hoops 126 times, and represented Scotland on 26 occasions.
Not bad for someone who will not celebrate turning 23 until July.
His best season for appearances was 2011-12, when he played 43 times for Celtic and three times for his country.
But the issues with his fragile frame were only in abeyance that campaign, and have since returned to blight his development, as his current sidelining - a groin strain picked up on Scotland duty has prevented him playing since March 1 - all-too-painfully confirms.
For all of this frustration, however, Forrest remains positive, and can look on the three championship medals and two winner's badges from the Scottish Cup to remind him that it has certainly not all been misfortune since he burst on to the scene.
Even in this staccato season, which might already be over for him as he continues rehab, there have been moments to savour, the most important of which was in the play-off match against Shakhter Karagandy when he popped up in the final minute to score the goal which took Celtic to the group stage of the Champions League.
"The Champions League was obviously the highlight, and winning the league as well," reflected the likeable lad from Ayrshire. "So, there have been a few highs."
He would be the first to admit, though, that there have also been too many lows.
Through it all, Lennon has remained his staunchest ally, holding him up as a beacon of hope for other kids who dream of making it at Celtic where the fail rate for wannabees is very high.
Forrest appreciates the backing he has had and said: "The manager has been here since I've been in the first team. I had him as reserve team manager, as well, and he has always been the same. He's been a great manager."
Lennon has recognised the importance of team spirit, so sadly and obviously lacking when he took over from Mowbray in March 2010.
He continues to make the club a happy and welcoming place to be, something kids stepping up from the ranks appreciate.
Forrest can see the likes of Henderson, O'Connell and Fisher benefiting from this in the same way he did and explained: "A lot of the first-team players now help the young boys. They talk to them a lot.
"So, when they come up, they feel wanted and good training with the first-team players. You play Under-19s. then you go up and train with the first team.
"If you do well there, you get a chance in the team - and you just want to take it. These three boys have done really well and have taken their chances."
Forrest can still recall how the nerves kicked in when he was told he was going to make his debut. But his goal soon made him feel at home as he wheeled away in delight in front of the Parkhead crowd.
"You want to score on your debut, and some of the other young boys have done that as well," he told Celtic TV. "There is no better feeling.
"You are nervous when you first go on, but, after a while, you realise you're just playing football, and that is what you have been doing for so long. You get used to it."
As Henderson and Fisher appear to have done, with O'Connell now following on.
Forrest has been very impressed by the latest graduates from the club's academy and said: "They've trained with us all season and are getting their chance now. They have deserved it.
"While we have had these three come into the side, there are others training with the first team, as well. Hopefully, for them, they get their chance, too."