Martin O'Neill believed Celtic would beat Barcelona after David Marshall saved a penalty and Chris Sutton levelled in a game that was lost 3-1.
"We had them on the rack for a spell and I thought we might go on and win this," admitted manager O'Neill.
"The penalty save galvanised us and we were on top after we got the goal.
"The players gave everything and their efforts have been brilliant over the last few seasons but the goals we conceded were poor."
Barcelona were on top throughout the first half and Marshall made a number of outstanding saves in the game, including stopping a penalty from Ronaldinho.
"They (Barca) are a very fine side with designs on winning this competition," added O'Neill.
"They passed it well, as befits a team of that quality. But we kept hanging in.
"They had some great chances but, when we did level, I thought they might settle for a draw."
The defeat was Celtic's first on home soil in the Champions League, while O'Neill's side have yet to pick up a point on the road.
"We've dropped three points," O'Neill continued. "Now we have go out and get something away from home."
Meanwhile, Jackie McNamara was carried off after catching his studs in the turf and O'Neill fears the defender may have a bad injury.
"It's not looking good," said O'Neill. "He's gone over on his ankle and he'll have a scan tomorrow."
In the other Group F match AC Milan picked up maximum points with a 1-0 win at Shakhtar Donetsk.