Barcelona v AC Milan: At what point does this dip for Barca and Lionel Messi turn to crisis?

The day after Barcelona lost 3-1 at home to Real Madrid in the Spanish Cup semi-final two weeks ago, Lionel Messi stayed in bed.

The club reported that he had a fever and was feeling unwell – so maybe a touch of man flu, or in Messi's case Superman flu. Whatever it was, it was gone in 48 hours and after an official rest day, the following day he was back in training before Barcelona's trip to Madrid.

It's not the first time Messi has been unable to face the world the day after a big defeat. Last season after Chelsea beat Barça 1-0 at Stamford Bridge and Real Madrid beat them in the league three days later, there was a similar no-show at training the following day, this time put down to gastric trouble. It makes him sick to lose.

Tonight that allergic reaction to defeat must drive him on against a Milan side – missing the European Cup-tied Mario Balotelli and the injured Giampaolo Pazzini – but sitting pretty with a 2-0 first-leg lead. They are on a run of 10 games unbeaten, and face a Barça team who have kept only one clean sheet in their last 14 games.

With such frailty at one end Messi might need to repeat last year's last-16 second leg performance at the other. He scored five against Bayer Leverkusen in a 7-1 win that made the aggregate score 10-2.

Barcelona beat Milan in the next round last season but then bounced off an unbreakable Chelsea – destined to win the big prize whoever they faced along the way. Messi and company feel they can learn from that second leg disappointment. They took a two-goal lead but allowed Ramires to score just before half-time and they never recovered. Tonight a clean sheet is unusually important hence "3-0 for Tito" having become the battle cry in the build-up to the game.

Tito Vilanova's absence through illness has coincided with the relatively poor recent run of form with six wins, three draws and four defeats in the last 13 games. There are good intentions in the home dugout as every match-day they leave the recuperating coach's seat free.

But the unoccupied place next to stand-in Jordi Roura has also served to reinforce the idea that the coaching team is weakened; that no one has stepped in to replace the temporarily unavailable Vilanova; and that the side is missing a leader.

Last night Gerard Pique picked up the standard with a press conference performance that brought Pep Guardiola's finest pre-match discourses to mind. He reminded everyone that the team are 13 points clear in La Liga and deserved to have people believe in them: "People have no memory. We have changed the history of this club in recent seasons of course we are capable of this [comeback] and much more besides." Asked about the criticism the side have received in recent days, he added: "The team will respond. Anyone who doesn't believe in the comeback should give their ticket to a friend."

Tonight's team-talk may well be given by Vilanova via video conference as it was ahead of the league clasico. He will also be picking the team, with Pedro and Alexis Sanchez expected to line up either side of Messi as Barcelona try to open up a well-watered Camp Nou playing surface as much as possible.

"The speed of our passing should be our weapon," Xavi said. "The better it is, the greater our chances." But former Barça "Dream Team" player Miguel Angel Nadal has warned against confusing speed with haste.

"If you think you have to score three then it is a mountain to climb," he said. "But if you concentrate on getting two then the third will come. You can't leave everything for one last big push but it has to be one step at a time."

Nadal was in the team that beat Dynamo Kiev 4-1 in the second leg of the last-16 in the 1993-94 season to come back from a 3-1 reverse in the first game. "This generation of players owe the Camp Nou a great comeback," said Xavi, who was part of the side that tried and failed to turn around the 1-0 Chelsea lead last season and the 3-1 Internazionale lead in 2010.

Other generations have delivered. Back in 1977 Bobby Robson's Ipswich Town took a 3-0 lead to the Camp Nou in the Uefa Cup. In front of just 25,000 supporters Barça won the second leg 3-0 with two goals from Johan Cruyff and went through on penalties.

They repeated the trick a year later in the Cup Winners' Cup against Anderlecht. And under Terry Venables in 1986 Barcelona came back from 3-0 down to win their European Cup semi-final against Gothenburg on penalties.

Xavi's call to arms has come accompanied by typically insightful tactical corrections of Barça's relative recent decline. "We have lacked the old movement in attack and the accompanying defensive intensity," he said. "Perhaps winning so comfortably in the league has caused us to drop our level a little.

"We need to stop conceding the free-kicks in wide areas that we are being punished from, and we need to start provoking fouls at the other end of the pitch instead." And above all he stresses the need to give Messi the space in which to best operate.

He added: "Against Celtic, Spartak Moscow and Benfica this season, the opposition have played two banks of four closing off all the spaces and stopping us playing our passes through the middle. We will try to open things up on both flanks so that the gaps start to appear again for Leo."

Despite the side's dip since the turn of the year and Messi's disappointing performances against Real Madrid and Milan in the first leg, only Cristiano Ronaldo has matched his goals scored this season. With 51 goals in all competitions and 40 in the league his efficiency has remained constant.

"We will have to play better than we did in the first leg to go through," the Milan coach, Massimiliano Allegri, has said, fully expecting something special from the Argentine in tonight's second leg. The four-goal salvo like the one against Arsenal in 2010 in the quarter-finals will do; a repeat of the five against Leverkusen would be even better.

"I don't think Leo Messi is sad," said the midfielder Cesc Fabregas this week. "It's just that when a player has won everything and he loses a game the next day they are still angry about it. But that's a good sign. That is what makes a team great."

No one at Barcelona wants Messi ringing in sick tomorrow morning.