Messi excuses beaten Barca
Goals from Victor Wanyama and Tony Watt at Celtic Park inflicted Barca's first Champions League group-stage defeat in three years.
The rare loss sees Barca's advantage at the top of Group G cut to two points and prevented Tito Vilanova's side from securing an early passage through to the knockout phase.
After the game, Messi reflected on a night where missed chances, the woodwork and another inspired performance from goalkeeper Fraser Forster cost his side the game.
"We did everything well, played better than two weeks ago against them, but the ball didn't go in," he told reporters.
"The same thing happened at the Camp Nou. They played defensively and they waited to make fast counterattacks. They are very comfortable on corners and set-pieces, and that’s how they scored their two goals."
Messi scored in the 90th minute and celebrated with a thumb-sucking gesture, five days on from the birth of his son, but it proved too little, too late, with the 25-year-old having earlier hit the bar.
Fellow forward Alexis Sanchez also struck the post.
"In the end, the goal didn't mean anything, the game was already lost. There are enough games left to dedicate goals to Thiago," Messi said, referring to his newborn son.