World Cup 2018: Lionel Messi looks stressed, which is a real worry - Pablo Zabaleta column

The happiest I saw Lionel Messi when I played alongside him for Argentina was in 2012 - he scored a hat-trick against Brazil in New York, and he played the whole game with freedom and a smile on his face.

His body language was completely the opposite before the Croatia game on Thursday. To see Leo looking so stressed and unhappy at this World Cup was a real worry.

He is my friend as well as my former team-mate and I felt really sorry for him. It is very unusual to see him like that, but it was a sign of what was going on in his head.

At times like this, there is a huge pressure on him because people expect too much, and for him to do everything for the national team.

I was not surprised he struggled, in what was a disastrous defeat for us. As a player myself, I know that if you go into a game and you don't really enjoy it then it is very difficult to play well.

  It was not just Leo, though, it was a poor performance by the whole team. We are very lucky that we still have chance to go through to the last 16 by beating Nigeria on Tuesday - but I am not sure it will happen. We will have to perform much better than we have done in our first two games and show the right attitude and much more desire. Football is a collective game and everyone needs to improve - not just Messi.This time, Argentina coach Jorge Sampaoli has to set up with the right formation and the right players, and follow a plan.

'Mascherano is the leader, Messi brings calmness, Aguero brings the music'

I have been there with this Argentina team when things have been going well - and when they haven't - and I know what the build-up will be like.

Sergio Aguero will make sure the right music is there for everyone. We always listen to cumbia, a traditional Argentine music, before the game.

It is similar to reggaeton because it brings happiness and the players love it, especially in the dressing room. That was what worked for us in Brazil at the 2014 World Cup.

In the dressing room, Javier Mascherano is the real leader. He is the one who is making the noise and speaking to everyone. But Leo brings calmness.

Leo is quiet and a person who does not talk much - but, of course, he is Messi.

Whenever I looked around the Argentina dressing room before a game, I always had the confidence I had the best player in the world on my team.

On Tuesday, in the moments before the game against Nigeria, the young players in the Argentina squad who are at their first World Cup will need to know they can follow these guys.

'Losing three finals in four years has been tough for that team'

I think it is getting harder for Leo to play for Argentina. He turned 31 on Sunday so he is getting older and sometimes, mentally, you can feel tired as well.

Part of that is down to his history with Argentina. This team has lost the past three finals they have played - the 2014 World Cup and the Copa America in 2015 and 2016.

In Argentina, that is not enough. For some people, and for parts of the media, you have to win - just reaching the final is not acceptable.

So losing three finals in four years has been tough mentally and psychologically for everyone in that team, especially Leo.

It is hard when people in your country treat you like a loser. They see him winning trophies every season with Barcelona - so they expect him to do exactly the same in the national team.

'Messi - always the main player, and the main man'

Like England, Argentina is a country that loves football so much and sometimes we cross the line in the way we treat our star players.

Big football countries have a massive passion and they expect the players to win and perform well. This is normal, but the big players feel it the most.

Leo gets criticised when he does not play well, but when he is not around - like when he briefly retired from international football in 2016 - there are people calling for him to come back.

As his team-mate, I can say without any doubts that we have always been a better team when he has been there.

And those who say he does not perform for his country like he does for his club are wrong. When I played in the Argentina team with him, he was always the main player and the main man.

I am sure he will be on Tuesday - because this could be his last chance.

'A time for unity - players and manager'

Along with Messi and Mascherano, my old Manchester City team-mate Nicolas Otamendi is part of the spine of the Argentina team, and is in control of the back four.

Those three will play a crucial role with their quality and experience, but Sampaoli must find a system that works.

I have not spoken to any of the Argentina players but I have heard the reports that the players wanted to get rid of him.

The formation he used against Croatia, a 3-4-3, was one he had used successfully with Chile, but it did not seem to suit the characteristics of our players.

He was working with a 4-3-3 before the World Cup, with only one defensive midfielder, but the day before the Iceland game he changed to play with two, even though we knew they would defend very deep.

Sampaoli also makes a lot of changes to his teams as well as his tactics, and even from the outside I can see how that would affect the atmosphere around the squad.

But now is the time for unity and I think they all need to sit down together - players and manager - because this kind of situation is normal in football in tough moments after bad results.

The players can listen to the manager - and they should also be able to express themselves about what they do not like from him.

If that happens, the players could really turn up on Tuesday. They have to if they really want to go through.

Pablo Zabaleta was speaking to BBC Sport's Chris Bevan in Moscow.