Perhaps they wouldn't admit it but Barcelona's hearts must have sunk slightly when Lionel Messi was forced off with a hamstring injury in the Champions League last week.

Having given the Catalans the lead with a sensational finish, his presence was missing in the second half as Barcelona had to settle for a 2 - 2 draw with PSG in Paris.

Messi is in the squad for today's return leg but whether he is fit enough to start at the Nou Camp is another issue entirely.

But will Barcelona adjust slightly if their mercurial false nine is forced to start from the bench?

The answer is probably no. When Cesc Fabregas came on he occupied a similar area to the one Messi staked in the diagram above as the roving false nine in the second half and he did score a fantastic hat-trick against a hapless Real Mallorca on Saturday.

But the rest of Barcelona's team need to step up in a similar way to the astonishing second leg against Milan. Barca have appeared lethargic when it comes to pressing in Champions League away games.

This could be a deliberate ploy to avoid getting trapped high up the field when the opposition counters, but it has not prevented any defensive issues as the opposition find it easier to expose the back four.

However, tonight Barca's midfield of Xavi, Iniesta and Busquets have one key factor on their side: no Blaise Matuidi.

The young French midfielder may have scored PSG's late equaliser last Tuesday but it was his contribution elsewhere that mattered most.

Sitting alongside the far less mobile David Beckham - who starts again tonight - it was Matuidi's role to engage Barcelona's midfielders directly, a role he fulfilled with flying colours.

But he picked up a yellow card at the Parc des Princes which rules him out of tonight's match.

Although Beckham range of passing was adequate against Barca last week, he will need an energetic partner to help him cover the ground with PSG academy graduate Clement Chantome tipped to start.

However Barca are missing a regular centre-half partner for Gerard Pique. Javier Mascherano picked up a knee injury in the first leg - and would have been suspended anyway - while club captain Carles Puyol is also nursing an injury.

Barca do have a few options like dropping Sergio Busquest back into defence although that would deprive the team of its best defensive midfielder. They could also play young defender Marc Bartra or play Alex Song alongside Pique.

Team shapes via UEFA.com