Celtic took a surprise lead when Charlie Mulgrew’s free-kick was met by Giorgos Samaras, with the Greek striker’s attempt deflected past Victor Valdes by Javier Mascherano.
Barcelona pressed for a leveller, and despite valiant Celtic defending, an intricate move saw the visitors’ back four carved open, with Andres Iniesta equalising in the 45th minute.
Lennon’s men dug deep in the second half and appeared set to secure a deserved point at the Camp Nou before Alba scored a dramatic winner in the fourth minute of stoppage time.
“There was a bit of luck with the goal,” said Lennon. “But we should have seen it out in the last 30 seconds, so while I’m bitterly disappointed for the players, I’m immensely proud of them as well.
“We were brave and the players stuck to what we asked them to do. We didn’t lose our composure when we knew we would be under pressure and we would give the ball away at certain times.
“We conceded ground when we had to but we defended brilliantly and at times on the counter-attack, we looked dangerous.
“This is the best team in the world, so to come here and play as well as that shows our quality.”
Lennon reserved special praise for Fraiser Forster, who produced a string of top saves to frustrate Barça throughout the Group G encounter.
“He’s quality, top class,” added Lennon. “He’s going to go really far in the game. I said he was going to have to be big for us tonight, and more or less every game since he’s come in, he’s been fantastic.”