QPR's hopes of securing automatic promotion look to be over after winning only two of their previous nine league games, but Karanka believes that they will threaten his side following their latest defeat on Tuesday.
The Spaniard told the club's official website: "QPR lost 3-0 against Sheffield Wednesday and played with 10 players after getting a player sent off.
"But if we think they will play like they did in the last game against Wednesday, we are wrong. We need to think of them as a team fighting for the promotion places.
"They were in the Premier League last season and have experienced players and a very good manager, and this makes it a very difficult game."
Middlesbrough are currently 13th, while QPR sit fourth in the table.