Belgium under-21 international de Sart has struggled for first-team action since arriving at the Riverside Stadium from Standard Liege 12 months ago, with his only Boro appearance this season coming in the EFL Cup defeat to Fulham in August.
The 22-year-old has now joined Derby on loan until the end of the season and he played the full 90 minutes of the Rams FA Cup victory over West Brom.
Karanka is pleased the defensive midfielder has joined Derby as he believes that is the ideal place for him to continue his development.
"Julien asked me my thoughts and I told him that the best thing for a player like him, young and a good player, is to play," Karanka said. "Derby is a team which I was pleased when I heard as they play our style, the manager, the club. It's a good move for him and it's a good move for Derby as he's a very good player with a bright future."
The Spaniard was also happy to allow back-up stopper Tomas Mejias to join Spanish outfit Rayo Vallecano on loan as he believes it is important the goalkeeper gets a chance to play regular first-team football.
He added: "Tomas is good for both of us, for him to play and to show he's a good keeper and if he does well he can come back. We have space for a goalkeeper, especially if they're playing and they're confident. That would be a headache for me."