Spaniard Karanka, who played for Real Madrid and was Jose Mourinho's assistant at the Bernabeu, is bidding to take Boro back to the Premier League via the play-offs.
But standing in their way are Brentford, the unfashionable west London club who have been the surprise packages of the Championship season.
"I hadn't heard of them," said Karanka
"But what matters is this season and this season they have played really well, they have good players and a good manager
I didn't need to know about them two years ago.
"This is the biggest game of my life, without a doubt
It is the first time that I have played for something.
"I have been here 18 months and the first season was just to know the team, and this season we were fighting until the last second and now we have to play the play-offs.
"For me it's a very good and important challenge because I have never been in a situation like this before.
"Of course I have experience as a player, and with Jose, and all are different, but I hope I am ready and I am loving every single second."
Karanka remains a close friend of Mourinho's and often turns to the title-winning Chelsea boss for advice.
Mourinho could even make the short trip along the Thames to take in the game, although when asked if he will be attending Karanka grinned: "I'm not sure."
He added: "We are in touch every week, we spoke after Chelsea's game against Crystal Palace to tell him my congratulations, when they became champions of the Premier League
Now they are at home cheering for Middlesbrough.
"I had been working with Jose for three years and when you have that relationship he is always giving me advice, because I have only been a manager 18 months and I need to learn every day.
"I will still talk to him next year
Friends are friends
But I hope I can't talk to him if we are playing him the following Saturday, as it means Middlesbrough will be in the Premier League."
Source : PA
Source: PA