Barcelona picked up two away goals in a 2-2 draw in the first leg of their UEFA Champions League quarterfinal against Paris Saint-Germain, giving them a great chance of progression to the last four, but lingering questions remain over the solidity of their defence.
It could well cost them a shot at final glory.
Lionel Messi, Andres Iniesta, Dani Alves, Xavi Hernandez—all are wonderful technical players who performed well on the night against PSG, even if Messi was quiet until he scored his goal.
However, all of them do their best work in the middle and final thirds of the field, where they defend from the front, pressing high up the field and maintaining possession in attacking areas. Further back, they are far from as effective.
Alves and his fellow attacking full-back, Jordi Alba, are expected to constantly bomb forward to provide outlets in wide areas. As a result, they are frequently out of position when teams counterattack against Barcelona, relying on their great pace to work back and win challenges.
Sergio Busquets, patrolling in front of the back "four," is a top-class player with excellent positional skills and no shortage of defensive technical attributes, but he alone cannot prevent every attack against Barça from culminating in a goal.
The central defensive partnership of Javier Mascherano and Gerard Pique has plenty of attributes of its own. Mascherano is physical, fast over the ground, strong in the tackle. Pique, meanwhile, is cultured, controlled, clever on the ball and able to break forward.
It is far from complete, though.
Take as an example the sustained pressure that PSG duo Thiago Silva and Alex came under at times during the match. The two managed to repel attack after attack, making blocks, tackles and above all else winning forceful aerial battles.
Neither Mascherano nor Pique have that in their armoury, and with Carles Puyol injured, there is certainly a gap in the skill set of the back line, which opponents will look to exploit.
For PSG, heading into the second leg at the Nou Camp needing a positive result to qualify for the last four, they need look no further than their own last-gasp goal on Tuesday night.
A high ball pumped toward Zlatan Ibrahimovic in the penalty area saw him win the ball almost unchallenged, free to knock it down for Blaise Matuidi to crack home the deflected equaliser.
A lack of real leadership, a habit of losing the ball in the defensive third at times due to perceived overpassing, the continued question marks over Victor Valdes and now the loss to injury and suspension of Mascherano—it all adds up to Barcelona facing something of a crisis at the back ahead of the second leg.
With Messi facing a possible spell on the sidelines through injury, Barcelona will not simply be able to rely on their free-scoring forward to shoot their way out of trouble. They are going to have to come up with a plan to defend against the likes of Ibrahimovic, Lucas Moura and Ezequiel Lavezzi.
And beyond that, should they make it, Bayern Munich and others lie in wait with even stronger attacking weapons at their disposal.
If Barcelona have serious designs on lifting this season's UEFA Champions League trophy, they need to sort out their lingering issues in defence, and quickly.