The 18-year old striker was introduced to the game with just under 20 minutes remaining and gave the Bhoys some breathing space when he slotted home their second goal in the 83rd minute.
Victor Wanyama had given Celtic a 21st minute lead at Parkhead and then Lionel Messi nabbed one back for the Catalan giants in injury-time after Watt scored and the youngster was absolutely thrilled to contribute in such a manner to one of the biggest wins in club history.
"It was the stuff of dreams for me, without a doubt. It could not have been any better for me," said Watt.
"It was my first goal here for the first team so it was really special.
"I saw the keeper coming out and I immediately knew I had to try and put it past him. Fortunately I did. It was great to score.
"I just took it in my stride and put it away. I had to treat it as I would've any other game.
"It's my job and I've got to put those kinds of chances away."
He added: "It's an unbelievable moment for me and everyone else at the club.
"It's a brilliant achievement for the manager. He picked his team perfect and he's brought me on and I've scored. Perfect."