The Spaniard has put pen to paper on a seven-year deal at Stamford Bridge, but Sarri acknowledged that the 23-year-old still has work to do if he is to live up to the standards set by Courtois.
"I'm very happy for both. The Courtois situation was clear. He wanted to go to Madrid, so we lost maybe one of the most important goalkeepers in the world now," Sarri told reporters.
"We bought a young goalkeeper, I think one of the most important amongst the young goalkeepers in Europe. I'm really happy. I'm really happy with Kovacic. He has the characteristics for us. I'm really very happy and I think now the squad is complete. Kovacic is a very technical player. He's able to play between the lines of the opponent. For us it could be very important."
When asked whether Kepa can be better than Courtois, Sarri added: "Not at the moment, for sure. He's very young, 23. I hope he can improve very fast. For the moment he's not Courtois, for sure."
Both Kepa and Kovacic could make their Chelsea debuts during their Premier League opener against Huddersfield Town on Saturday.