Boro have now won five out of their last six in the Sky Bet Championship and have kept five clean sheets along the way in a marked upturn in form.
They sealed victory at the Riverside with a long-range effort from Emmanuel Ledesma in the 16th minute but were left hanging on nervously in a frantic finale.
Karanka said: "I was very happy when the final whistle went because I was really suffering there at the end.
"It was another three points and another clean sheet so I must be satisfied - but it was not a good game.
"We did not play as well as we can or as well as we should so I was not happy at that. We did not move the ball quickly enough and we did not press our advantage when we were on top of the game.
"But it was a game that we may have lost in the past and it is another win so that is a good thing.
"And it is a big lesson for us. It has showed how difficult it is to win games in the Championship, how hard you must work to get in front and how hard it is to stay there."