Marco Ambrosio likened the defeat of Arsenal in the Champions League to climbing K2. The Chelsea goalkeeper was entitled to consider it a steep task, considering his club's 17 previous games without a win against Arsene Wenger's team, but mountains remain treacherous on the way down from the emotional peak and the team will have to tread carefully against Monaco tonight.

Chelsea hit an emotional high a fortnight ago but now the mood has altered. The manager Claudio Ranieri calls all of the contestants in the last four "surprises", but some put his team in a separate category. "This will be a difficult game," said the captain Marcel Desailly, "because everyone in England thinks we are already in the final."

This is a new pressure for Chelsea, and Ranieri himself had no cause to be serene yesterday. The bug that has been roaming the Harlington training ground has now affected Damien Duff and William Gallas to the extent that neither could even travel.

The manager cannot now rule out selecting Juan Sebastian Veron, despite his fairly recent back operation. "He could play anywhere," Ranieri said of the level of challenge that would face the midfielder this evening. "He is an amazing player, but I have to know he is 100 per cent fit."

The only current evidence Ranieri can weigh came in a 33-minute spell against Middlesbrough 10 days ago. Chelsea drew then and have not won any of their three matches since beating Arsenal. He will have to trust that lesser aims have been reduced to cinders only because the burning desire for the Champions League has incinerated them.

Chelsea, with results in Europe that would normally make them sure of themselves, are full of imponderables. There is no particular indication that the manager will be allowed to keep his job and in the climate of instability the players, as Ambrosio suggested, still have to confirm their future under the club's owner Roman Abramovich.

"The boss wants the best for Chelsea," the goalkeeper said of the Russian billionaire. "We have to show him that Chelsea have the best players now and that he doesn't have to change a lot over the next few years. To do that we have to try and win the Champions League."

That enterprise has been at its most impressive outside London, and Chelsea, at the Stade Louis II, can extend a perfect away record in the tournament this season with a sixth consecutive win. After the Abramovich investment, there is always a great deal that can go right and Hernan Crespo, an unstoppable scorer for Internazionale in the Champions League group last season, believes his "moment" has come with Chelsea.

If the striker truly is sharp again, Ranieri will be all the more regretful that a key provider has been lost. Much as he talks of the need to hit the net here, Chelsea will depend on the sturdiness that has given his team its exemplary defensive record, with a mere five goals conceded in this year's competition. Monaco, all the same, have the prolific streak to wreck such a record, having hit eight against Deportivo La Coruna in a single game and notched five over the two legs of the quarter-final in which they eliminated Real Madrid. Though much attention has been paid to the centre-forward Fernando Morientes, Ranieri highlights the captain Ludovic Giuly by comparing him to Gianfranco Zola.

The Chelsea manager, bombarding Monaco with compliments, portrays the side as a fusion of French style with Italian tactics. It is a description engineered to flatter Didier Deschamps, the French coach who once played with such distinction for Juventus. He was also at Chelsea, of course.

This is a tie festooned with reminiscences for each club and, in particular, for two men who triumphed together with France in World Cup and European Championship. Deschamps collected a second Champions League medal with Juventus, and his great friend Desailly got his at Milan. Only Clarence Seedorf, a victor with Milan last season, has won the Champions League at three different clubs, but Desailly could equal that feat.

"You can't be sentimental," said Deschamps of Monaco's desire to cast Chelsea aside. "I wouldn't be depriving Marcel of anything. It's a competition."

Desailly will assuredly have to work hard tonight if there is to be any prospect of reaching his target. With Jerome Rothen on the left to complement Giuly on the other wing, Monaco should sustain pressure. Both John Terry and Claude Makelele have slight knocks but surely cannot be spared action when they and Desailly may be the keys to the outcome of the tie.