In the 18th minute Georgios Samaras' header from a Charlie Mulgrew free-kick went in off the back of Barca's Javier Mascherano to give the visitors a shock lead but a minute before the break Barca midfielder Andres Iniesta levelled.
With the four minutes of added time almost played at the end of an exhausting evening in which Parkhead keeper Fraser Forster had been outstanding, defender Jordi Alba broke Hoops hearts when he sneaked in at the back post to knock an Adriano cross past Forster from close-range.
The Celtic boss said: "I thought the team were magnificent. It is bittersweet really, it is hard to take losing the game in the manner in which we did but I am so proud of the players, I think they gave the club and country a huge shot in the arm.
"There is a mixture of disappointment and anger that we didn't see it through to the end of the game.
"The players are hurting, they are angry, whatever you want to call it but it gave them a real a taste of playing in the Champions League at the highest level and they played fantastically well.
"No one gave us any hope but I think we have won a lot of admirers.
"They were brave, I couldn't ask any more from them, maybe just 30 seconds more to see it through.
"It would have been fantastic to pick up something from this game but we didn't budget for anything from it.
"I hope they take plenty from the game. It is tough to play at this level.
"But there are no recriminations and no criticism. I think we leave with our reputation intact."
Celtic remain second in the group with four points from three games ahead of the return game against Barcelona in a fortnight's time.
Lennon believes his side still have a chance of going through to the knockout stages of the competition.
The Northern Irishman said: "You have to look at the three performances so far, we have done very well.
"There are three games to go so anything can happen.
"I don't think they will take a lot of picking up, they can take a lot of satisfaction from this performance.
"The next game is Barca at home and we have to be mindful of that.
"We don't want to go gung-ho but we will have a big crowd and that will spur us on."