Not a bad one for Leigh Griffiths either. This was not the sort of bludgeoning Neil Lennon's side have doled out to various, cowering sides in recent times but the solitary-goal victory was good enough to lift them 21 points clear of Aberdeen at the top of the SPFL Premiership.
More significantly, it was an 11th league clean sheet in succession for Forster, beating a club record which had stood since 1922. The Englishman has now kept his goal intact for 1035 league minutes and is closing in on Edwin van der Sar's world record of 1311 minutes.
This would have been one of the easiest clean sheets recorded by Forster throughout that run but he still had to do a bit to earn it, the Celtic goalkeeper making an impressive stop around the hour mark to deny Josh Magennis a goal on his St Mirren debut. That aside, the goalkeeper could have slung a hammock between his posts and had an afternoon siesta so rarely was he tested, his achievement testimony to his powers of concentration as much as his agility between the posts.
"These are special times for the players," said Lennon. "When they look back on their time here, they can reflect on this as quite an achievement. To break such a long-standing record is absolutely fantastic."
At the other end of the field, however, this turned out to be an afternoon to forget for Celtic as they passed up chance after chance to make the game safe. Kris Commons' strike after five minutes seemed to set them up to go on and bury St Mirren under a flurry of goals but they were unusually wasteful, firing shots high and wide or into the arms of goalkeeper Marian Kello as Danny Lennon's side dug in doggedly.
With new signing Griffiths left watching from the stand, ineligible to feature after completing his move from Wolverhampton Wanderers on the last day of the window, this was not a day for Anthony Stokes to continue his profligate streak. His most glaring miss came in the final minute. Sean Kelly was dispossessed on the halfway line following a St Mirren corner, allowing the Irishman a clear run towards goal with no defender near him.
He could have shot early or passed sideways to give Derk Boerrigter a tap-in but instead elected to round Kello, going so wide it was as if he was trying to circumnavigate a tanker. "He should have squared it at the end," added Lennon.
"It's about decision-making. He only had to roll it to Derk and it was 2-0 and game over. That was a bit disappointing."
Lennon was more impressed by the performance of Stefan Johansen, describing it as the best debut he had seen in his time as manager.
The Norwegian looked tidy and enterprising in a deep-ish midfield role and the obvious candidate for the man of the match award. "There is more to come from me when I get my full match fitness," he said. "In the last few minutes I was tired. I hadn't played a match since November 15 - I just had two subs appearances and a reserve match. I am happy with my performance but I need to work even harder to stay in the team."
That Celtic would probably break that clean sheet record was apparent from around midway through the first half as St Mirren struggled to gain any sort of foothold in the forward areas. John McGinn, impressive throughout, shot wide after just 13 seconds but it would turn out to be among just a handful of chances created by Danny Lennon's side as they succumbed relatively tamely. The St Mirren manager, though, praised the efforts of his players.
"Celtic had the lion's share of possession as they always do but we competed with them," said Lennon. "We went toe-to-toe and kept bums on seats. It's probably up there with as well as we've played here."
This was a welcome return home for Neil Lennon following his visit to Tynecastle the previous day when he was subjected to verbal abuse, spat on and had coins thrown at him as he took in the League Cup semi-final between Aberdeen and St Johnstone. The Celtic fans put a figurative arm around their embattled manager, standing to afford him a round of applause in the 18th minute of the game while chanting his name.
The Celtic manager stepped out of the dugout to acknowledge the support. Almost inevitably, it was Commons who sent Celtic on their way to another win.
Emilio Izaguirre was the creator, picking out the forward around 20 yards from goal from where he drilled in a low effort for his 22nd goal of a stellar personal season.
Celtic were comfortable thereafter but not clinical. The best first-half chances came just before the break. Forrest was denied by a smart save, while Stokes - who had put an earlier chance over the top - was again wasteful after being picked out by Charlie Mulgrew.
Celtic continued to batter away, with Forrest sclaffing a shot past the post and Mulgrew bludgeoning another high into the stand, before Stokes had an effort saved by the goalkeeper, headed wide, then missed that late sitter.
One goal, though, would prove to be enough on this occasion.