The January transfer window has opened and with it the temptation for players to go looking for an upgrade. But for those who are hoping to shine in this year's World Cup, here's a cautionary tale.

Just under four years ago I made a quick trip to Paraguay to speak to Anibal Ruiz, the Uruguayan-born coach who was then in charge of the national team. His side started well in qualification, but then hit a sticky patch in the middle before coming good towards the end of the campaign with the introduction of some new blood, especially playmaker Julio Dos Santos.

A lot like Juan Roman Riquelme

Tall, elegant, with a touch of Argentina's Juan Roman Riquelme in his languid stride, Dos Santos was Paraguay's player of the year in 2005, wearing the No. 10 shirt of local giants Cerro Porteno.

The eyes of Ruiz shone when he talked about him. Dos Santos, he said, was "different class." This was the player who would provide the touch of quality necessary to take Paraguay to the World Cup quarter-finals for the first time ever.

I asked Ruiz if he was not worried about the fact that Dos Santos had moved to Germany to join Bayern Munich at the turn of the year. Not at all, I was told. The player had received a big money offer from the Middle East. The fact that he had chosen Bayern instead showed that he was giving priority to sporting criteria rather than financial. This was a good sign.

A couple of months later the club season came to an end and the teams that qualified for Germany came together to prepare for the World Cup. Paraguay played some warm up friendlies - and there was unwelcome news for Ruiz. Dos Santos looked terrible. He was a shadow of the player who had been so impressive the previous year. Off the pace and out of touch, he seemed incapable of making an attacking contribution. And as he did not have any defensive strengths, his coach was left with a problem. Ruiz was building his planning around Dos Santos. But the player was in such awful form that he was hardy worth his place in the side.

Ruiz left him out of the opening game of the World Cup against England, and Paraguay went down to a tame 1-0 victory. With half an hour to go in the next match, against Sweden, Dos Santos came off the bench. Paraguay, though, were caught in two minds. They made attacking substitutions while also seeming happy with a goalless draw. Late on they were punished for their indecision.

Dos Santos had come on for left sided midfielder Cristian Riveros. The Swedes intelligently brought on Johan Elmander to operate in the space that Riveros had vacated, and with two minutes to go his cross set up the game's only goal. Paraguay was out, and its third game, against Trinidad and Tobago, was of strictly academic interest.

Now it was too late, Dos Santos was in the starting line up and had a hand in both goals in Paraguay's 2-0 win. But it was nothing more than a taste of what might have been. In effect, the loss of form that Dos Santos suffered, directly related to his move to Germany, sabotaged Paraguay's planning and undermined their World Cup campaign.

Long past his best

Sadly, almost four years on Dos Santos has still not recaptured his 2005 level of performance.

Joining Bayern set him on a downward spiral. An alien culture, not being selected, being just one of the squad rather than the centre of attention - it all conspired to rob a promising career of its momentum. Bayern loaned him out - with no success at all - to Wolfsburg and to Almeria in Spain. He then came back across the Atlantic to Brazil, where he played with little distinction for Gremio and Atletico Paranaense. And last year he turned full circle, rejoining Cerro Porteno, where sometimes he plays and sometimes he is on the substitutes bench.

Now approaching 27, he should be entering his peak years. There is still time for him to come good again - but he will surely have to produce something special in this year's Copa Libertadores if he is to force his way back into the national team squad in time for the World Cup in South Africa.

By now, Paraguayans would have hoped that Jose Montiel would be pulling the strings in the centre of the national team's midfield. He showed immense promise at youth level, so much so that he was taken to the last World Cup for experience at the age of 18. Straight afterwards he moved from Olimpia, Cerro Porteno's historic rivals in Asuncion, to Udinese in Italy, a club who specialize in sweeping up young South American talent. There was one season there, one with Reggina - with hardly a chance to get on the pitch, a spell in Romania with Politehnica Iasi and he ended the year in Argentina, on the bench for Tigre, the club who finished bottom of the first division. The hopes he generated have emphatically not been matched - and it's hard not to see the premature more abroad as the villain in this process.

Of course, the young player is by no means the only one responsible for these unwise decisions that can have such a detrimental effect on his career. Usually there is an agent dreaming of his cut. And very often his South American club is desperate to make the sale - they need the money to pay off the wage bill from three months ago.

But the player's anxiety for a quick move is also part of the problem. What if he gets injured? What if he loses his form and no one wants to buy him in the future? The chance is now - and for today's youngsters, accustomed to a world of instant solutions, that is a very hard temptation to resist.