The 23-year-old Spanish global has become the most expensive goalkeeper in history after completing a £72m move to Stamford Bridge.
Madrid's starting goalkeeper has been Keylor Navas, who helped the club win the last three Champions League titles.
Castles told the Transfer Window podcast: "Kovacic has been offered to Chelsea to facilitate a deal for Courtois".
Kepa has always been heralded as one of the best young players in the World, and he is very much a modern 'keeper who is equally adept with both his hands and his feet.
"His long-term contract reflects the belief we have in him".
NYC votes to stop Uber's unchecked growth, give drivers a minimum wage
The first such cap by any major US city was part of a package of measures that also includes setting a minimum wage for drivers. Debt and financial hardship have been blamed for the deaths of six taxi and car-service drivers in the past year .
Chelsea have announced the loan signing of Mateo Kovacic from Real Madrid.
Chelsea confirmed earlier today: "Chelsea and Real Madrid have tonight agreed terms for the permanent transfer of Thibaut Courtois subject to the agreement of personal terms and passing a medical".
Kepa is Spain's number two behind Manchester United's David de Gea.
Breaking the bank for Kepa is a drastic change of direction for Chelsea, having lost out to Liverpool in the race for Brazil No. 1 Alisson earlier this summer due to their reluctance to meet Roma's valuation of the player, who ultimately moved to Anfield for £56m.
Chelsea FC are looking to at least break back into the top four this season after they finished fifth and won the FA Cup under Antonio Conte last term.
Athletic Bilbao has already confirmed that Spaniard's release clause of £71m has been paid.