Negredo joined Boro over the summer and lacked real competition to his spot in the starting lineup over the first half of the campaign.
Karanka was unhappy with the Spaniard’s performances and questioned his work ethic after starting the term positively.
The Boro boss convinced Gestede to sign up from Aston Villa in January and Karanka insists it is no coincidence Negredo’s form has taken an upturn.
Karanka said: “He is working well.
“It is not a coincidence that with Rudy here now, there is more pressure on him.
“You could see in the last 10 minutes when Rudy was on the pitch (against Everton), he touched the ball three times and had one chance.
“And now Alvaro feels that pressure – and that has to be good for us. Presure like that is good for everyone.”
Motherwell held Ladbrokes Premiership leaders Celtic to a goalless draw despite playing for more than half the match with 10 men at Fir Park.
Michy Batshuayi's cheeky backheel was enough to give Borussia Dortmund a 1-0 win over Hannover 96 at Signal Iduna Park on Sunday, lifting them within a point of second-placed Schalke.