So that's what the fuss was all about. When Lionel Messi walked off the pitch in Paris there was fear; when he walked back on again in Barcelona there was hope. He had spent eight days struggling to get fit and 50 minutes chewing his nails on the bench, a huge roar accompanying him as he emerged to warm up. Now he had half an hour to save his side. It took nine minutes. Wembley is in sight once more: for this club, no stadium carries such symbolic weight. No player does either.

It is easy to forget that Barcelona's decisive goal against Paris Saint-Germain was scored by Pedro, too simplistic to overlook another decisive contribution from the Canary Islander. But Messi's introduction changed the atmosphere at Camp Nou on Wednesday; his team-mates talked of an impact that was psychological as much as physical, for them and the opposition.

Down in the underground car park, team bus awaiting him, David Beckham wore a resigned look as he insisted that he felt Paris Saint-Germain really could win it, only for the game to shift. "Messi is the best player in the world – it's as simple as that," Beckham said.

Everyone was talking about Messi after the match, Barcelona's assistant manager, Jordi Roura, calling him inimitable, and everyone was talking about him the following morning too. When he pulled out in Paris, there was a kind of panic, an impotence that crept over everyone, a cold terror.

A 5-0 victory over Mallorca at the weekend suggested that they could live without him but that paralysis persisted. The talk had been of Messidependencia. Here, that was brought sharply into focus. Even injured, he was vital. As if they were lost without him.

It is an exaggeration, certainly. After all, Spain have been the best international team in history without him and there were eight from the selección in the starting XI. But on Wednesday there was something in it.

The recurring word is cojo. Look it up. It'll say crippled, maybe even lame. Crocked. Even cojo, he is the best. "One leg is enough for Messi," claimed the headline in El País. "Messi unlocks Pedro," said AS. "Messi reigns – even when crippled," ran the headline in Marca. The cover of El Mundo Deportivo called it "Messi spirit." Inside, they wrote: "And Messi flicks the switch." The bench had been a cage where "he prowled, going round in circles, smelling his prey but unable to get at it."

It had been five years since he started a game on the bench and of the last 59 Champions League matches Barcelona had played, he had been involved in 56. They had hoped that he would not be involved this time; the risk was one they would have preferred not to run but in the end they had to.

"Let's not debate Messidependencia any more," Ramón Besa concluded in El País, "his mere presence is enough. One day, a cardboard cutout of Messi will win a match."

But Messi's impact disguised the failings of a Barcelona side who will have to improve dramatically if they are to reach the final. It seems absurd to seek flaws in a side who have just become the first team to reach six successive European Cup semi-finals but they are there. Absurd too to question a midfielder who completes 100% of his almost 100 passes as Xavi did on Wednesday but there was little of the control he has exercised in the past. Nor could Sergio Busquets dominate as he has often. Only Andrés Iniesta performed at his normal level.

Barcelona had only two shots on target all night – their lowest total since Opta started compiling Champions League statistics in 2003. Víctor Valdés's seven saves were the most he has made in 43 European games. Zlatan Ibrahimovic, dropping deeper, dragging Gerard Piqué with him and turning playmaker, created six opportunities. That was his highest total too. He may feel that PSG could have made more of his generosity.

There is a vulnerability about Barcelona, a lack of security. Even with Messi on the pitch, there were concerns. They finished the game with Alex Song replacing David Villa and going into the deep midfield role alongside Busquets: the famous doble pivote looked down upon by football's purists, but necessary here. Much needed protection amid the nerves. Until the final whistle, they were not safe.

By then, it was clear that Messi was not quite ready either, although he is sufficiently intelligent to manage a match and his own physical limitations. He now has a fortnight to recover fully for the semi-final. That is their greatest concern but not their only one. There is reason to believe that Barcelona will continue to struggle. There could hardly be four stronger teams in the semi-finals; all three potential opponents would cause them problems, particularly in light of their absentees.

Javier Mascherano has a knee ligament injury and faces five weeks out. Carles Puyol picked up an injury on 15 March and was told it would be six weeks before he is back. Song was supposedly a defender too and may yet be one but that experiment did not last long and does not convince yet. The coaching staff remain unsure about youth teamer Marc Bartra. Adriano is a left- or right-sided attacker, midfielder or defender who played at centre-back for the fourth time this season – and promptly picked up yet another muscular injury. Even if he does recover in time, he will be suspended for the semi-final, first leg.

Eric Abidal played for Barcelona last Saturday for the first time in 401 days, having recovered from cancer and a liver transplant. Beyond the human impact of an astonishing story, there is a footballing question too: Barcelona could do with him. But he is not ready yet: his doctor insists it is a miracle that he is playing at all.

And that is pretty much that: there is no one else for the centre-back role. Piqué is still standing but the prospect of an injury to him is terrifying. Simply put, they are a team who do not have defenders. But they are also a team who do have Lionel Messi.