Swansea had started with surprising urgency against a Boro side still reeling from their midweek mauling by Liverpool, with Modou Barrow missing an early opportunity and Gylfi Sigurdsson bringing a save out of Victor Valdes.
But Boro shrugged off the early loss of Viktor Fischer with a knee injury and clawed their way back into the game, Negredo bringing a save out of Lukasz Fabianski before he opened the scoring in the 18th minute with a fine curling shot after a cross from Adam Clayton.
Boro were still not entirely convincing despite the returning Ramirez – a major influence – almost making it 2-0 when he fired a free-kick just over the bar after being fouled by Angel Rangel in a dangerous position.
It was perhaps no coincidence that Boro lost the two games that Ramirez missed with a foot injury, and Karanka admitted: “Gaston has made a big difference since he arrived here last season.
“When he is not on the pitch the team can’t feel him, but I didn’t want to use that as an excuse after the last two games because we win and lose as a team.”
Negredo scored his second from the penalty spot just before the half-hour after Jordi Amat clipped Adam Forshaw in the area, and de Roon polished off the simplest of chances from a cross by Ramirez on 58 minutes.
It spoke volumes for the paucity of Swansea’s threat that Karanka should pronounce himself dissatisfied with his team’s performance despite a convincing win that leaves them clear of the relegation quagmire.
Karanka also praised the resurgent Negredo, who has swatted some early criticism since scoring two goals in the 2-2 draw at Leicester at the end of last month.
Karanka said: “He (Negredo) needs to have those chances to score. He scored against Leicester and has had chances in other games but he knows adding goals is very important for his confidence. But for the other players on the pitch, even when he is not scoring goals they feel that he is contributing to the team.”
Middlesbrough: Valdes, Barragan, Chambers, Gibson, Da Silva (Traore 87), de Roon, Clayton, Forshaw, Ramirez (Friend 62), Negredo, Fischer (Downing 12). Unused substitutes: Bernardo, Leadbitter, Rhodes, Guzan.
Swansea: Fabianski, Rangel, Mawson, Amat, Taylor, Britton (Fer 70), Fulton, Barrow (Borja Baston 59), Sigurdsson, Routledge (Montero 77), Llorente. Unused substitutes: van der Hoorn, Nordfeldt, Cork, Naughton.
Referee: N Swarbrick (Lancashire).