De Sart signed for Middlesbrough from Standard Liege on deadline day, after the club’s scouts spotted his potential.
The 21-year-old is yet to make his Boro debut, but Karanka is pleased to have the youngster available to him for the rest of the season and beyond.
“Julien is a midfielder and the best thing is he is our player, he is not a loan like Bruno [Zuculini] or Jack [Stephens] so for that reason I am really pleased," the Spanish boss said.
“He is young, he is talented and he is going to have really good future.”
De Sart was one of five additions which Middlesbrough made in January - alongside Ritchie De Laet, Jordan Rhodes, Gaston Ramirez and Kike Sola.
The Belgian is arguably the least known of the players who have been brought in, but Karanka has obviously seen enough to be impressed.
Breaking into Middlesbrough’s team in the near future could be a tough ask, for the midfielder, however.
Boro are currently in second place in the Championship table, and they will move to the top of the standings if they are able to beat Milton Keynes Dons later this evening.