Ex-Liverpool and Manchester City star Dietmar Hamann has pinpointed Borussia Dortmund's defence as their weak spot ahead of Tuesday's UEFA Champions League showdown with Celtic.

Celtic travel to Germany full of confidence after their perfect start to the season continued with a 6-0 hammering of St Johnstone in Perth.

They can expect an altogether different challenge in their second game of the new-look format to Europe's premier competition against last season's Champions League runners-up.

But Dortmund have looked vulnerable at the back having conceded seven goals in their last two games following a 5-1 defeat at Stuttgart and a comeback 4-2 win against local rivals Bochum.

And Champions League winner Hamann - who also played for Bayern Munich and Newcastle United - has highlighted Dortmund's problems.

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He said: "Borussia Dortmund appear to be all over the place at the back at the moment.

"There was no organisation to their defending. No one seems in control.

"I got nervous just watching them trying to defend.

"BVB must improve significantly over the course of the season both in the league and in Europe.

"The Stuttgart debacle posed a lot of questions about them defensively. And to be honest, I don't think we will get many answers this season."