Arjen Robben's potential move to Real Madrid could end up being an expensive deal for the Spanish champions, according to coach Bernd Schuster.
Madrid are understood to be ready to pay £20million to land the Holland international.
They have also drawn up a salary package, including image rights, which the Dutchman would accept, but Chelsea coach Jose Mourinho insists he wants the winger to remain at Stamford Bridge.
Robben has two years left on his contract and Chelsea will demand at least £25million if he continues to refuse to sign the new five-year deal he has been offered.
Schuster, who is also chasing Feyenoord wide man Royston Drenthe, admits that negotiations for Robben could turn out to be very difficult and involve a hefty fee.
The Madrid coach says he would be much happier if the deal had already been concluded but maintained that Madrid were prepared to go right to the wire in their chase for the Chelsea player.
He said: 'It could be expensive. It is necessary to go slowly. We need to be prepared to make signings until the final day of the window.
'I would be much happier if they were here. The signing of Robben is very important for us.'
Robben wants a six-figure contract at Chelsea but the offer on the table is for £90,000-a-week - considerably less than top earner John Terry's £135,000-a-week deal and the salaries of other leading stars Andriy Shevchenko and Michael Ballack.
Chelsea are set to increase the pressure on him to make his long-term intentions clear before the start of the season but the Blues are willing to play a waiting game with Frank Lampard.
The England midfielder also has two years left on his contract and has refused the club's offer.
However, his agent, Steve Kutner, insists the stalemate will not affect his form.
Kutner said: 'If it goes all the way to the last day of Frank's current contract, one thing's certain, Frank will not waver in his commitment to Chelsea.'
Mourinho wants the England international to pledge his future to the club but the midfielder is still not entirely happy with the terms of the contract offered to him by the club.
It has increased speculation in Europe that Barcelona will test Chelsea's resolve yet further by making another attempt to land him.
However, Chelsea's veteran midfielder Claude Makelele is ready to sign for a further year after looking almost certain to leave during the summer.
The France international found it hard to hold down a first team place last season after a disagreement with Mourinho over the player's decision to continue turning out for his national side.
But Makelele's experience will be vital for Chelsea as they attempt to wrestle the title back from Manchester United and the midfielder appears to be on the brink of staying at Stamford Bridge for a little longer.
Chelsea's unbeaten pre-season run came to an end against Rangers on Saturday and they now travel to Denmark to play Brondby tomorrow night.
But winger Shaun Wright-Phillips insists the defeat at Ibrox has already been forgotten.
Wright-Phillips, who replaced Joe Cole at half-time in Saturday's 2-0 defeat, is keen to look forward to the challenge in Copenhagen.
He said: 'I just want to get it out of my head to be honest. That's it now, we're on to the next game and we just have to prepare and play well.
'I think every game we play is good preparation. We need to use it for gelling and fitness, and getting results to get our confidence up.'
Chelsea prepared for their final pre-season game before the FA Community Shield at Wembley with a training session that also included left-back Ashley Cole.
However, missing from the same session were John Terry (toe) and Robben (knee) along with Andriy Shevchenko, Paulo Ferreira, Makelele and Michael Essien.