Hazard has been linked with a £100million transfer to Real Madrid at the end of the season after finding his top form again in 2016-17 under Antonio Conte.
The attacker scored twice in Wednesday's vital 2-1 win at home to Manchester City, a double that moved him on to 13 Premier League goals this season and edged his side closer to a second title in three years.
And Courtois, who made a rare mistake for Sergio Aguero's goal in the match at Stamford Bridge, is in no doubt Chelsea have to resist any approaches for Hazard, however lucrative they may prove to be.
"Yes [Chelsea have to keep him]," Courtois told the Evening Standard. "Eden is one of the best players in the world.
"Chelsea have ambitions to win the Champions League again and always look to play for the title.
"If you want to do that, you need the biggest players and he is obviously a big part of that."
The goalkeeper added: "Eden is a great player and showed it again against Manchester City. It was a great first goal.
"His penalty was from the rebound, which reminded me a bit of what happened against Crystal Palace when we clinched the league two years ago. Hopefully it will go the same way.
"He is fantastic, but he works hard for the team too and did a lot defensively, which is what we need from him. I am very happy for him."
Courtois himself has been linked with Madrid, but reiterates he – like Hazard - remains happy at Stamford Bridge after achieving a milestone against City.
"We are both very happy to be here," said Courtois. "This was my 100th game for Chelsea and I hope for many more.
"Eden is not affected by it. They are just rumours. He is just calm. He knows that he is important to Chelsea, like I do."