Arsenal have a mountain to climb when they enter the Nou Camp on Tuesday bidding to reach the semi-finals of the Uefa Champions League. Barcelona, the defending champions, dominated the first leg of their quarter-final clash at the Emirates last week but were rocked by two late goals to leave the tie balanced on a knife edge at 2-2. Emmanuel Petit, who played for both clubs, tells BBC Sport how the Gunners can pull off a shock win in Catalonia. THE FIRST-LEG FACTOR The north London side were outclassed for much of last Wednesday's match before their late rally. Petit admits he was surprised to see his former side dominated in such a fashion. "Arsenal were outplayed for an hour by a much better team, at least technically in terms of passing, possession and chances to score," the 39-year-old former French international said. "I've never seen them outplayed like that before." Despite the opening half ending goalless, official Uefa statistics show the extent of Barca's imperious onslaught. The Spanish visitors produced 279 successful passes to Arsenal's 98, with 15 shots and seven of them on target, to the home side's two, which were both wayward. "Barcelona's first 30 minutes against Arsenal was the best I've seen them play for a long time," added Petit. "Maybe they feared Arsenal and had prepared themselves to respond to their quality. In terms of possession of the ball and technically, Barca were a better team. When Arsenal saw that, they stepped back." It was a case of the guns being turned on the Gunners, who are used to dominating other teams, particularly domestic opposition. Against Wolves on Saturday, Arsenal claimed over 60% of possession and fashioned 13 shots to their opposition's two, according to BBC statistics, as they beat the Premier League strugglers 1-0. "Arsenal have to improve and show that what we saw in the first half at the Emirates was not the way they normally play," said Petit. "But Barca have the advantage of two away goals and the way they outplayed Arsenal for an hour last week will give them the confidence to do it again." ARSENAL'S MIDFIELD CONUNDRUM Arsenal will be without their injured captain Cesc Fabregas. While Petit admitted the loss of the Spanish star is a blow, he suggested it could actually help them. "I've seen them play very well without Fabregas," said Petit. "When Fabregas plays, he is always asking for the ball and every ball comes through him. That's quite easy for an intelligent team to defend against, as we saw against Manchester United, Chelsea and Barcelona. "If you stay true to your philosophy - keeping the ball on the ground, movement, passing, speed - then it doesn't only depend on one player." Petit also pointed out that Arsenal boss Arsene Wenger still has plenty of alternatives when it comes to replacing the former Barcelona player. "Samir Nasri is better when he plays in central midfield," insisted Petit. "He has stepped up physically and mentally. He realised what he needed to do to play at the top level. He has improved a lot over the last few months and I'm sure he can play in central midfield." BARCELONA'S DEFENSIVE DILEMMA Barca are without captain Carles Puyol, who is suspended after conceding the penalty from which Fabregas equalised last week. It is a big blow for Barca, who are also without Puyol's regular central defensive partner Gerard Pique. He is also banned after picking up a second booking at the Emirates. "Puyol and Piquet missing the game is good for Arsenal," said Petit. "Pep Guardiola has had less than a week to put a new centre-back pairing together and that could be the biggest chance for Arsenal. "Rafael Marquez and Gabriel Milito will probably play in central defence, and that could allow Arsenal to do something because those two don't play together often. "Those positions are very important for Barca. It's the spine. They may not have the automatism." WENGER'S BIG DECISIONS Arsenal also have huge defensive problems, with William Gallas and Alex Song both unavailable because of injury. Petit feels the loss of French international Gallas could be significant. "Gallas is very important in central defence as we saw from both goals Barca scored in the first leg," he said. "Sol Campbell could be the man to replace Gallas in defence because they need some experience in there, someone who can organise and talk a lot." England winger Theo Walcott came off the bench in the first leg to score Arsenal's first goal, and Petit urged manager Arsene Wenger to pick the England winger from the start. "They need Walcott because the pitch is huge, the space is huge," said Petit. "There will be a lot of one-on-ones. As soon as Arsenal get the ball, they need to find Walcott or the other winger straight away. "Striker Nicklas Bendtner will provide the physical presence with his aerial threat and pressure on the centre-backs. Walcott will provide the pace. It's a good mix." NULLIFYING THE BARCA THREAT Barca's record at the Nou Camp this season is astonishing. It reads: 22 games in all competitions, 59 goals and 13 conceded. However, Guardiola will be without injured striker Ibrahimovic, who scored both Barca goals in the first leg, and the Barca coach admits he is "short of players". So what should Arsenal's tactics be? "If they want to win the return, first they really have to defend very, very well," said Petit. "Arsenal have to start very differently to how they did at the Emirates. I saw them running, but running where? They didn't put pressure on Xavi or Sergio Busquets. "Every time they got the ball it was too easy for them to start passing. If you don't put pressure on the holding player, then it's very difficult to defend. "Possession will also be key. If Arsenal cannot keep the ball and play the way we know they can, they stand little chance of qualifying. In the first half of the first leg, it was about 70% possession to Barcelona and 30% to Arsenal. It was like the students meeting the master." But the Nou Camp crowd, used to watching such a consummate team, can be fickle - and that can be to Arsenal's advantage, according to Petit. "If Barca are in trouble, the fans can get frustrated very, very quickly," said the former Barca midfielder. "They can show their anger. Barca also have big games coming up in La Liga, including next weekend's El Clasico against Madrid. That could play on the players' minds. "It all depends on how Arsenal start. If they start well, everyone watching in the stadium and on TV will quickly realise it's not the same game they were watching last week. "Confidence can play a huge part and the Barcelona public are not that confident and put pressure on their players very, very quickly." SPEED AND PHYSICAL PRESENCE Despite Barcelona's phenomenal record at home, they are by no means invincible. They lost 2-1 at home to Russian side Rubin Kazan last October in the Champions League and were undone by the same score by Sevilla in Spain's domestic cup competition three months ago. "Arsenal have to be very, very intelligent tactically," Petit claimed. "For me, they have to play on the wings. "The likes of Lionel Messi and Pedro won't defend, so a lot of space will be created on the wings. That's where Arsenal have to play. "When they get the ball they have to go quick, quick, quick. Barcelona hate speed. They want to control the game technically and tactically." Petit also says the Gunners will need to dominate Barca physically. "Don't try to play too much football," he said. "The teams who have done well against Barcelona play direct and at speed, as we saw in their first leg against Stuttgart in the last round. "Barcelona were outplayed because of the speed and physical presence. Passing on the ground but quick, minimal touches of the ball. Barcelona hate that. "Don't think Barcelona cannot be beaten - they have been beaten this season. They have been in trouble against teams who put physical presence into their games." Story by David Ornstein Additional writing from BBC Sport's Phil Dawkes and Mark Ashenden