The former Manchester City man moved to Boro in the summer on a season-long loan deal from Valencia and scored on his debut against Stoke.
However, the Spain striker has failed to net since and has now gone six Premier League games without a goal.
The 31-year-old had to settle for a place on the bench against West Ham before the international break and Karanka has indicated the striker could again be among the substitutes when Watford visit on Sunday, with Jordan Rhodes likely to start.
Karanka says, at his best, Negredo is one of the league's top strikers but feels he is still getting used to his new surroundings after his switch from the Mestalla.
"I am here to try to take that pressure off. I knew for him, it was going to be difficult," said Karanka.
"He started really well, but sometimes it's normal when you start really well and you feel really, really confident, you start to be a little bit more relaxed and you lose your position.
"For that reason, he has to fight. He is fighting a lot on the training ground because he failed against West Ham, but at this club, nobody is bigger than the team.
"He is learning because it's important for everybody to know that the main thing here is the team.
"But playing his best, I can compare him with [Christian] Benteke or [Romelu] Lukaku and the better strikers in the league."