De Laet has joined Boro from Leicester on loan until the end of the season, strengthening their defensive options.
And Karanka is delighted with the addition of the Belgian, as he didn’t believe it would be possible to do a deal with Leicester.
“With Ritchie it was the same as David Nugent at the start of the season in that I didn’t think he would be available to join us,” Karanka said.
“Ritchie is an experienced player who knows the league and was promoted with Leicester.
“He knows the league and how to play with this kind of pressure. He knows how to play in the Premier League, too.
“He’s also a really good lad which is the main thing.”
De Laet started the season in Leicester’s first team, but he has dropped out of Claudio Ranieri’s starting plans more recently.
The 27-year-old has not made a Premier League start since September, and Leicester decided that allowing him to leave the King Power Stadium temporarily would be beneficial.
De Laet joins Middlesbrough as they look to consolidate their position in the automatic promotion spaces.
Boro are currently only one pint behind league leaders Hull City, and they have a game in hand over many of their promotion rivals.
De Laet could make his debut for Middlesbrough in their fixture at the weekend against Blackburn Rovers.