Both teams went into the tunnel at half-time with the score tied at 0-0 and without registering a shot on target.
However, Karanka's talk during half-time worked wonders, and the Spanish manager was happy with the commitment showed by his players in the second-half.
The Boro progressed into the next round with a 3-0 win over their Championship opponents, thanks to goals from Grant Leadbitter, Alvaro Negredo and Marten de Roon, despite losing Daniel Ayala to a red card in the second half.
Following the game, Karanka said: "The second half was completely different.
"I told them at half-time that I thought we were playing a friendly game and for me, the FA Cup is not a friendly game, it's a really important game.
"Sometimes it's difficult when you go on to the pitch against a team in a lower division to get the right mentality and in the second half, we went on to the pitch with the right attitude, the right mentality and even when we lost Dani, we were really consistent.
"Everybody was committed, everybody was fighting for every single ball and that's the main thing for me."
When, asked whether he was angry with his side's performance in the first-half, he said: "Yes, because the image that we were showing wasn't the right image. They came here knowing that they are in the Championship and they wanted to show everybody that they can compete against us.
"Now in every single football, if you don't go on to the pitch 100 per cent, you will have problems."