The Bluebirds went down 4-2 on home soil to Norwich City at the weekend, with the Welshmen imploding after leading 2-0.
Boro, by contrast, enjoyed a morale-boosting late victory at Huddersfield Town and a win tonight would see them climb into the top six.
But head coach Karanka is wary and fully expects Ole Gunnar Solskjaer’s side to be in the shake-up come season’s end.
He said: “The most important thing is to start well, always, and that will be the same at Cardiff.
“Against Norwich, they were defeated, so they will want to show everybody the kind of team that they really are.
“For this, we have to be prepared, we can’t think it is going to be an easy game. If we think it will be easy, then we will make a very big mistake.
“The Championship is a very difficult division and I don’t have doubts that Cardiff will be high in the division.
“They have a very good squad, a very good group and a good coach.”
Karanka reports no fresh injury problems ahead of the trip to South Wales.
Tomas Mejias (back) is still sidelined, with Dimi Konstantopoulos to continue in goal, with Jamal Blackman will be on the bench.
Summer signing James Husband (calf) is still missing with a calf problem.
Last six games: Cardiff WWLWDL; Middlesbrough LWLWLW.
Referee: G Scott (Oxon).
Last time: Cardiff 1 Middlesbrough 0; Championship, November 17, 2012.