Rodgers was unhappy with his side going forward and defensively

Celtic "lacked aggression" in defence and were "too negative" in attack as they exited the Europa League to Zenit St Petersburg, said Brendan Rodgers.

The Scottish champions headed to Russia in an optimistic mood after an impressive 1-0 victory in the home leg.

But familiar defensive failings resurfaced as Branislav Ivanovic, Daler Kuzyaev and Aleksandr Kokorin scored in a 3-1 aggregate loss for Rodgers' side.

"Defensively, we lacked aggression, first half especially," he said.

"We didn't close the space quickly enough. We talk about 'defending forward' and we just didn't do that well enough - and, of course, we conceded poor goals.

Ivanovic was left unmarked in the box to powerfully head home a corner after seven minutes to level the last-32 tie.

The second came when Daler Kuzyaev's swerving 25-yard shot deceived Dorus de Vries and Aleksandr Kokorin bundled in Ivanovic's cross to make the result safe.

"The corner is something that can happen. Ivanovic is brilliant in the air and we got blocked off," added Rodgers.

"For the second goal, we didn't engage quickly enough from the throw-in. We were too deep and had to press the guy shooting.

"At 2-0 down, we still had a chance, but everything was too sideways and backwards. It is about having the bravery to play.

"Second half, we started OK - but again we conceded possession too cheaply for the third goal."

Celtic conceded three poor goals from a defensive point of view

Celtic enjoyed 67% possession overall, but as their manager acknowledged, "possession is no good on its own; you have to penetrate and there has to be creativity with it".

"We didn't have enough control in the first half in order to play forward," Rodgers added. "We didn't threaten at all because we were too negative in our play.

"We weren't looking forward quickly enough and didn't get into the final third with any quality.

"But, to really threaten, you have to ensure you defend better and the third goal was a very disappointing goal to concede."

After finishing third in a Champions League group that included Paris St-Germain, Bayern Munich and Anderlecht, Celtic achieved a target of remaining in Europe into the new year.

And, despite their Europa League campaign ending in the round of 32, Rodgers believes there are signs of progress.

"We were much better than them in the first game," said the Northern Irishman. "In this one, if we had played with a bit more courage and belief, we maybe could have got a result.

"There's still an awful lot of work for us to do, but taking a step back, there have been a lot of positives for us in both European competitions.

"We have played very well in some games and shown some great qualities. Of course, we are up against the best of the best, so there was a lot of learning taking place.

"There have been aspects that have pleased me and we have taken steps forward with a lot of young players.

"It is probably just consistency. Last week was an outstanding performance, which would allow you to think we came into this game with a great opportunity.

"But if you don't start the game with that aggressive mentality then it is really difficult."

Celtic will now turn their attention to the pursuit of another domestic treble. They resume their Scottish Premiership campaign against Aberdeen at Pittodrie on Sunday and play Greenock Morton in the Scottish Cup quarter-finals on 3 March, having already retained the Scottish League Cup.

"We have to go away and finish our domestic campaign as well as we possibly can and try to create another piece of history on that front," Rodgers added.

"Then we have to look to be better in Europe again, as we were this year, and try to take another step forward."