Even after last season’s humiliating 7-0 aggregate defeat against eventual European champions Bayern Munich, Barcelona seem not to be getting it right in terms of strengthening their squad ahead of the new season.The reigning Spanish champions are one of the most respected clubs in the world following their recent success in both the Champions League and Spanish League, especially under Pep Guardiola, the man who won 11 trophies in the three seasons he was in charge.The Catalan side is known for their offensive style of play and midfield domination. Yet they are still dogged by a weak defence, which Bayern devastatingly exposed last season en route to claiming their fifth Champions’ League title after an 11 year drought.One doesn’t need to be a rocket scientist to realise that Barcelona’s weakness is their backline which includes offensive-minded wing backs in Dani Alves and Jordi Alba.These two highflying wing backs normally leave their central defence exposed to the opposition attack as seen in the Bayern Munich loss where Robben and Ribery became very difficult for Barca defence to handle.Barca defence has for a long time been reliant on services of Carles Puyol and Gerald Pique as their experienced stoppers. However, age and injuries have limited Puyol’s involvement while Pique remains highly prone to blundering in the absence of his vastly experienced skipper.In their absence, Barcelona central defence is left in the hands of midfielders-turned-defenders Mascherano and Alex Song, who, with due respect, are not real defenders and are prone to many mistakes.The Catalan side should either sign a central defender in the remaining weeks before the transfer window closes on August 31 or change their system of play.The change of playing system is difficult based on their tick-taka style that makes them stronger than their opponents.Many expected Barcelona to make a central defender their top priority this summer but attracting a suitable man in that area is proving to harder than many expected with Thiago Silva committing to PSG, while Chelsea are holding out for the highest possible price for Brazilian David Luiz.The other remaining options that could match Barca’s performance level are Manchester City’s Vincent Kompany  and Borussia Dortmund’s Mats Hummels. But it is very difficult for their clubs to release them.The club has also been affected by change of management, especially due to illness of now-former coach Villanova. It will take time for new coach Martino to get used to the Catalans style of play.And with the signing of Brazilian playmaker Neymar from Santos this summer, Barca seem destined for offensive football again like in recent seasons.With Messi, Neymar, Xavi, Pedro, Sanchez and Iniesta, Barcelona have probably the most fearsome attack in Europe; yet without a solid backline, some opponents, who can defend well and at the same time attack with intent, will have a good chance against them.