It was a night for exclamations, and the teenage striker was lost to the reveries of the moment. "It was the stuff of dreams for me," he said, as though still trying to make sense of the occasion.
Watt displayed poise as he raced through on goal then kept his composure to slot the ball beyond Victor Valdes. The 2-0 lead looked secure, only for Barcelona to pull a late goal back through Lionel Messi. Like his team-mates, Watt was "nervous" as he waited for the final whistle, but Celtic did not let the result slip. Afterwards, the striker paid tribute to the inspiration provided by the manager, Neil Lennon.
"It's a brilliant achievement for him," he said. "He's picked his team perfectly, and obviously he's brought me on and I've scored. Just perfect. His team talk was inspirational, but then it always is. It's got to be one of the best victories in the history of the club."
Every Celtic player contributed, and Fraser Forster made two outstanding saves to deny Lionel Messi, before eventually being beaten by the Barcelona striker. The victory puts Celtic in a strong position in Group G, with their next assignment away to Benfica.
"We need to keep the momentum going," Forster said. "I don't think anyone will really remember that game in the Camp Nou now, they'll just be enjoying [this victory].
Messi was less than gracious after the defeat, and complained that the result did not reflect the fact that Barcelona enjoyed more possession of the ball than Celtic. "It reminded me of the first match," Messi said. "All that was missing for Barcelona was luck."