The Teesside club, fourth in the Championship, can temporarily go top of the table if they prevail at Norwich, where they have won just once in 40 years.
Boro head into the game after back-to-back home wins over Wolves and Rotherham United, but mindful that on their past two televised matches on the road, against fellow promotion rivals Watford and Bournemouth, they have been well beaten.
Karanka is under no illusions that Boro cannot afford a re-run if they are to keep their top-two hopes alive.
Asked if he felt the losers would miss out on automatic promotion, Karanka said: “I think so because everybody is expecting Bournemouth, Norwich, us and Watford to lose a game, but nobody is losing games.
“So with two games to go, five or four points below the (automatic) promotion position would make things very, very difficult.
“We will go to the game confident we can beat them, but if we think that it is going to be easy because we played well against Wolves – no, it will be very different.
“I think the loser can forget direct promotion, but the winner won’t definitely be in the Premier League either.
“The winner has to keep fighting until the end because there are four teams fighting for automatic promotion. I think it’s very important to win because one point will not be enough.”
The fitness of Jonathan Woodgate, Albert Adomah and Daniel Ayala will be assessed today. Grant Leadbitter is back from suspension and competing for a place with Adam Forshaw.
Last six games: Norwich DWWWWW, Middlesbrough WLWLWW.
Referee: R Madley (West Yorkshire).
Last time: Norwich 1 Middlesbrough 0; October 23, 2010; Championship.