Hazard, 27, has two years left on his current Stamford Bridge deal and he wants to leave the club. Chelsea did not have a great campaign in the Premier League last term as they finished in fifth place.
In Castles' mind, there is only one option.
Real Madrid are the huge favourites to sign both players with their odds as low as 1.33 to sign Hazard and just 1.20 to sign Courtois.
It was announced in 2017 that Donnarumma would not be renewing his contract with Milan, and while a few weeks later he did sign until 2021, that has done little to ease rumours about his potential departure.
After Liverpool's recent acquisition of Alisson for £66.8m, their transfer dealings could very well be done for the summer, which could path the way for Chelsea to splash out on Fekir.
Real Madrid hold a long-standing interest in Hazard, with former manager Zinedine Zidane also showing his keenness to sign the Belgian global during his tenure.
"I don't think they can get that money from Real Madrid".
"It's a way of football that I believe in, that I really like, that I grew up in", Fabregas said.
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"They have a big stadium renewal project at the Bernabeu which is priced at €400m - that has to be funded".
It is yet to be seen whether the youngster can emulate the recently departed Madrid legend, Cristiano Ronaldo. This time feels different, however, as reports continue to intensify regarding the signing of Chelsea's Courtois.
Cesc Fabregas this week pleaded with Hazard not to quit Chelsea. We all know that I've always felt well at Chelsea, so.
When asked at a news conference whether he has spoken to Hazard, Fabregas said: "I won't lie, I speak to him often". For that you need your best players.
He's a player that we all love to play with, especially me.
Hazard has been one of the most pivotal players in the Chelsea squad, progressing leaps and bounds since his arrival from Lille in 2012.
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