World Cup injury concerns headlined by Neymar and Mohamed Salah

It wasn't just 200 million Brazilians who breathed a sigh of relief when Neymar came off the substitutes' bench to score for his country in Sunday's 2-0 friendly win against Croatia at Anfield.

Having been sidelined with a foot injury since February, the Paris Saint-Germain forward showed the watching world that not only was he fit and ready for the World Cup, but he also has his shooting boots on too.

Neymar's return is a big deal for Brazil, but it also crucial for Russia 2018 that the star player of the most glamorous team in the tournament is all set to it the ground running when it begins next week.

World Cups need the best players performing on the biggest stage, which is why Lionel Messi's rescue act for Argentina against Ecuador on the final day of qualifying in South America (when the Barcelona forward scored all three goals in a 3-1 win) was celebrated in all corners of the globe.

A World Cup without Messi or Neymar would be a diminished tournament, so from the powerbrokers at FIFA down to the kid playing football on the street, it is only a good thing that both of them are fit and ready for the big kick-off in Russia.

Only time will tell whether Neymar is fully fit and firing on all cylinders in time for Brazil's opening game against Switzerland in Rostov on June 17, but the 26-year-old's goal scoring return at Anfield has at least confirmed his fitness and lessened the sense that Brazilian coach Tite is taking an almighty gamble by selecting the world's most expensive footballer.

But there are other world stars who are heading to Russia with question marks over their ability to play, too. Some of them carry the hopes of a nation, so the pressure on the player and coach will be intense as the days tick down to the opening games.

The likes of Laurent Koscielny, Dani Alves and Alex Oxlade-Chamberlain have already been ruled out due to long-term injuries, but the world is still waiting to see whether Mohamed Salah can overcome his shoulder injury to play for Egypt, or if Vincent Kompany's groin injury will eventually force him to watch Belgium's campaign from the bench despite being named in the final 23.

There are also questions over the fitness of Argentina's Sergio Aguero after he missed the final six weeks of Manchester City's season with a knee injury and England captain Harry Kane, whose ankle injury clearly affected the Tottenham forward during the final two months of the season.

Kane appeared to banish the doubts with an impressive goal scoring performance in England's 2-1 win against Nigeria at the weekend, but Aguero's 31-minute run-out for Argentina against Haiti last week is unlikely to be enough to persuade Jorge Sampaoli that he is fit to start against Iceland in Moscow on Jun 16.

And what about Germany goalkeeper Manuel Neuer, who only returned from a foot injury sustained in September during the 2-1 defeat against Austria on Saturday?

Salah is the big doubt, however, after suffering his injury during Liverpool's Champions League final defeat against Real Madrid in Kiev on May 26. Despite posting upbeat bulletins on social media over the weekend, the winger is facing a race against time to be fit for the game against Uruguay in Ekaterinburg on June 15 -- Egypt's first World Cup finals fixture since 1990.

Egypt are a major force in African football and their 28-year absence from the World Cup has finally come to an end, so the pressure on Salah to be fit was already intense before his injury. But he must now overcome a problem that could rear its head again due to the short period of recovery time between Kiev and his country's opening World Cup game.

Egypt coach Hector Cuper is experienced and smart enough to know the risks attached to gambling on Salah's fitness, but it is often the downside of coaching at a major tournament.

Tite has the same problem with Neymar, but even though the player had not kicked a ball in a competitive game for over three months before Sunday, his recovery time has been such that he will have confidence that his star man is primed for action.

However, there are two recent examples of star players returning from injury at a World Cup only to see their gambles backfire.

In Neymar's case, the story of Michael Owen at the 2006 World Cup should be a warning.

Owen also suffered a foot injury ahead of the World Cup, with the then-Newcastle forward playing just one game for his club after being injured on Dec. 31, 2005. He went on to play in two World Cup warm-up games and start three games in the tournament before suffering a serious knee injury against Sweden in England's final group game, which has since been attributed to Owen not recovering the full strength in his legs during his rehabilitation from the foot injury.

Wayne Rooney also travelled to Germany in 2006 with his fitness under a cloud, having suffered a broken metatarsal while playing for Manchester United at the end of April.

England coach Sven-Goran Eriksson defied Sir Alex Ferguson and took a chance on Rooney, who was unable to start the first two games due to his lack of fitness. Rooney eventually made four appearances at that World Cup, but failed to score and ended the tournament with a red card for stamping on Portugal's Ricardo Carvalho.

Neither Owen nor Rooney were fit for action in 2006 and England suffered the consequences.

But it takes a brave coach to leave a star player at home to get fit, and even braver player to tell an expectant nation that he is not ready to play.

Neymar, Salah, Aguero, Kompany, Neuer and Kane will all be on the plane. But only time will tell if they are ready for this World Cup.