When the spot-kick was given at the end of the first half, the weight of the world rested on Forrest's shoulders. The referee took a while to issue bookings, he made sure the ball was repositioned on the spot, goalkeeper Jasper Cillessen attempted some timewasting: there were plenty of distractions which could have filled Forrest's head. Instead, nothing did. He took the kick with perfect serenity, burying it high and hard in Cillessen's net.
So what made it even more painful for a Dutch team to concede such a goal, considering their national team has gone out of four World Cup or European Championships on penalties? Forrest scored without practising.
No-one would suspect it, from the aplomb of his finish, nor from the fact the only previous kick he took - in a 5-0 defeat of Dundee in February - also went in. "No, I don't practise. Not at all. Sometimes before cup games the boys will take a few in training, but other than that I don't. I've still always said I wanted to take one in a game though."
What Forrest has is an uncluttered, nerveless mind under pressure and the confidence that comes with having taken many successful penalties as a youth player. "Obviously it's different taking penalties for the under-19s. There's no crowd, no pressure. It's different when you're out there in front of 60,000 fans. The Ajax goalkeeper was trying to play mind games but it maybe just made me concentrate even more. I just stayed focused on what I was going to do with the ball once everything had settled down. Nobody actually said anything to me as I was waiting to take the kick, but the way the defenders and the goalie were acting they were doing everything to try to put me off.
"Kris Commons has scored so many - his record is really good - so when he comes back he'll probably still be the regular penalty taker. I'll step aside if he wants back on them."
Forrest's goal ended a sequence in which Celtic had failed to score in four consecutive games in the Champions League proper: two against Juventus last season and the opening Group H ties against Barcelona and AC Milan. The fact that Milan, in second, have already dropped four points means the group remains tight. Barca lead on seven points, Milan have five, Celtic three and Ajax one. Celtic now face the Dutch again in Amsterdam on November 6.
"I think we're just going to take it one game at a time," said Forrest. "We'll look to the trip to Amsterdam next, then take it from there. I am confident we can win in Amsterdam. Hopefully we'll get a couple more boys back [Commons, Scott Brown and Adam Matthews were missing] so we're only going to get stronger. We'll have a good chance."