The Hoops were made to pay for early profligacy in the first leg of their third-round qualifier less than two minutes into the second half when Rasmus Schuller fired the visitors ahead. However, striker Gary Hooper levelled in the 54th minute before defender Charlie Mulgrew headed the winner seven minutes later.
The Celtic boss was bullish in his assessment of his side's first competitive 90 minutes of the season and their prospects in the return game in Finland next week. He said: "I am not disappointed at all. It is a huge improvement on where we were last year. I think Helsinki can count themselves pretty fortunate that it was only 2-1."
The Northern Irishman added: "They will think they have got a good result but I seen enough there....there is plenty of scope for us in the away tie. I am not looking at it through rose tinted glasses, I do look at it from a different perspective sometimes. I am pleased with the performance.
"The scoreline doesn't reflect the performance but sometimes that happens. We have a week to get ready, we have another game at the weekend obviously and the players will benefit from that game tonight.
"The tempo of the game was good and some of the football and chances created were superb against a team who just wanted to sit in and counter-attack.
"We want European football badly and I think the players showed that. For me, the tie is in the balance. We are a goal up which gives us a slight advantage over them.
"They will have to come out and score, they showed they are capable of doing that but then again, so are we."
Lennon was especially pleased by the way his players reacted to losing a goal just after the break. The former Celtic skipper said: "We showed brilliant character, the game could have got away from us at that stage."
HJK Helsinki coach Antti Muurinen admitted his side got the away goal they wanted. He said: "The result mattered most and we have a good chance in the second game. I was especially pleased to get the goal away from home and I was especially pleased with the way we played in the first 45 minutes."