By Simon Austin

Kalou does not fulfil the criteria for a work permit

Salomon Kalou will certainly not be Chelsea's biggest signing of the summer - but he may turn out to be the best.

The 20-year-old Ivorian striker signed a four-year deal with the Stamford Bridge club on Tuesday and arrives with a formidable reputation.

He scored 35 goals in 67 games for Feyenoord from a wide position and won the 2005 Johan Cruyff award for being the best young player in Holland.

Yet Feyenoord assistant manager Johnny Metgod has an ominous warning for Chelsea's rivals - that Kalou will improve with age.

"His individual skills are amazing for someone of his age," former Nottingham Forest and Holland midfielder Metgod told BBC Sport.

"But he is not the finished article and can improve a lot. The thing he needs to do is to keep his head up and assess the best option.

"He has great ball skills and his instinct is always to beat a player. Yet it's not always best to go for the one-on-one, especially if you are in your own half.

Salomon is not a typical English centre forward - he is very, very light footed and extremely quick Johnny Metgod Feyenoord assistant coach

"You need to keep possession of the football, not lose it. It will be good for him to get more experience.

"He will learn where he can play his own game and where he has to adapt."

Kalou has played as a left-sided striker for Feyenoord in a 4-3-3 line-up - the same formation that Chelsea have employed under manager Jose Mourinho.

But Metgod thinks it will still take him some time to adjust to Chelsea's style of play and to English football.

"Salomon is not a typical English centre-forward," he said. "He is very, very light-footed and extremely quick - certainly over the first few yards - but is not that strong yet.

"The experience will be good for him in England and I certainly found him to be a quick learner. This was only his second year with Feyenoord and he adapted very quickly."

With the impending arrival of Andriy Shevchenko at Stamford Bridge and the fact that forwards of the calibre of Didier Drogba, Arjen Robben and Shaun Wright-Phillips are already at the club, Kalou is not assured of a first-team place next season.

Indeed, he is not even sure to get a work permit and might have to be loaned out to another European club - perhaps in Belgium, which has more liberal nationality laws - in order to gain EU citizenship.

Kalou is uncapped, so he does not fulfil the criteria required to gain a work permit.

WORK PERMIT CRITERIA Automatically issued if: Player has taken part in 75% of his country's games over last two years AND country ranked 70 or above in the world Appeals process: Club can argue player's case before football review panel, which then submits recommendations to work permits department.

Chelsea are refusing to comment at the moment but they could claim mitigating circumstances were he refused a permit.

The reasons are complex.

Kalou arrived in Rotterdam - where his older brother Bonaventure, the current Ivory Coast captain, was playing for Feyenoord - in 2000.

Although Bonaventure moved to Auxerre in 2003 and then on to Paris St Germain, where he still plies his trade, Salomon stayed in Holland and signed for Feyenoord in 2004.

Ivory Coast coach Henri Michel was keen for Salomon to join his older brother in the international set-up but instead the player elected to apply for Dutch citizenship after Holland coach Marco van Basten reportedly promised him a place in his World Cup squad.

Dutch immigration minister Rita Verdonk refused to allow the player to be fast-tracked for a passport and, after a series of court cases, the appeals process was exhausted and he failed to gain citizenship.

It means he will be watching the World Cup from home - and that Mourinho might have to wait to utilise Kalou's prodigious talents in his Chelsea team.