IF Celtic under Brendan Rodgers are to ever come close to once again becoming an established European team, even one with realistically modest aims, then they must learn how to defend.

Every goal is, of course, avoidable but this side over the past 18 months have made it far too easy at this level for the opposition to score at will and Zenit St Petersburg, a good but not great team, were in no mood to turn down such opportunities in Russia.

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Celtic do not have the resources to compete with the best or indeed, with clubs several notches down from the elite; however, it is not asking a lot for these players to stop conceding from corners, free-kicks and shots from outside the box.

This is why the Scottish champions are out of Europe after yet another chastening night away from home and the domestic scene. It was more than a touch depressing.

Not since 2004 have Celtic played in Europe beyond the first knock-out round and, to be brutally frank, that’s not going to happen again for some time until they improve at the back.

Rodgers must look at bringing in a goalkeeper and at least one, probably two, central defenders this summer or what was witnessed in St Petersburg and also Paris, Munich, Barcelona, Israel and Astana – they scored four even if Celtic did get through – is going to happen again and again.

It’s not good enough. No Celtic supporter expects their team to win a Champions League but they must be fed up watching the same mistakes.

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The St Petersburg Stadium, to give the magnificent arena it’s official name, is reportedly the most expensive in the world. There was little change out of £1.5billion, and much of the money was spent on the temperature control.

It was freezing outside and, yet, almost balmy inside, which added a surreal element to the evening. Alas for Celtic, the outcome was all too familiar.

Rodgers went with the same team from last week, and the same 3-5-1-1 formation in the hope they could replicate what was by some distance Celtic’s best performance of the season. They did not. By an equal distance.

Although, at the start Celtic actually looked the part. They got on the ball, passed it to each other with pace and accuracy, which always helps, and with a confidence that has not always been there in their European travels.

But any team is only as strong as their weakest link Celtic have a real problem defending set-pieces. This lack of basics cost them dearly yet again.

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With seven minutes gone, and this was the first time Zenit had done anything in the way of an attack, the ball was worked to Leandro Paredes on the edge of Celtic’s box and the £20m summer signing’s shot brought out the best in Dorus De Vries who got down low to push the ball wide.

And from that corner, Zenit scored a simple goal.

True, the cross from Matias Kranevitter was good, but former Chelsea man Branislav Ivanovic found it so simple to get above the rest and power a header past De Vries who had little chance of doing anything about it.

The scorer was Jozo Simunovic’s man. What has happened to a player who was so reliable?

How many times now have Celtic conceded a straight forward goal in Europe? Too many, is the answer, and with the tie level they found themselves in a position they never looked like getting out of.

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Within moments of the opener, Mikael Lustig failed to cut out a pass to Zenit’s No9 Aleksandr Kokorin who for a split-second looked to be clean through but in the end his finish was weak.

Was it going to be one of those long nights when Celtic spend an hour and a half with their backs against a wall? It wasn’t quite like that but even at this early stage there was a feeling of inevitability about what was to come.

Paredes is a player and on 19 minutes his free-kick, which was a good 25 yards out, was only just over the bar. The Russians, however, were two goals to the good soon enough.

The shot from Daler Kuzyaev on 27 minutes was decent enough, the midfielder certainly got power and swerve on it from outside Celtic’s box, but De Vries completely misjudged the flight and he went one way while the ball went the other and into the net.

So that was two goals Zenit did not have to work for.

For Celtic, there was nothing going on up front. They passed the ball okay, but for the rest of the half never hinted that they could score a goal of their own. Indeed, while Zenit are no world beaters, their passing and crossing almost all the time caused Celtic trouble.

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Tom Rogic replaced the anonymous Eboue Kouassi at the break and the Australian at least had a crack at goal within a minute of the second half beginning, albeit it was always going over. And, in fairness, Celtic did have more about them.

All they needed was one goal, so it wasn’t as if there was no hope. But it was all over just after the hour.

With Zenit’s first attack of the half, the ball broke to Ivanovic, he sent in a low cross and three Celtic defenders, plus De Vries, couldn’t get close to the ball but Kokorin, who got in front of Lustig, did and he just managed find the back of the net from close range.

Olivier Ntcham at least forced a save from Zenit goalkeeper Andrei Lunev, the shot went straight to him, on 67 minutes, as did Rogic but the game and tie was gone from Celtic and they exited European football in the end with a whimper.

And nothing will change unless that team is strengthened.