Boro held the defending champions to a 2-2 draw on Saturday and would have claimed all three points had it not been for a last-gasp Foxes equaliser.
Negredo, on-loan at the Riverside from Valencia, scored twice for Karanka's men as he ended an 11-match goalless run stretching back to the opening round of fixtures.
There had been calls from some sections of the Boro fans for Jordan Rhodes to be given a chance as Negredo struggled in front of goal, but Karanka insists he always believed the Spaniard would get back among the goals sooner rather than later.
"You know I have to make decisions and sometimes I am criticised, but I have to think about the team and I know Alvaro," he said, as reported by the Gazette Live.
"I know how they train every day and I always try to put the best XI on the pitch.
"I always said that Alvaro is a top scorer. He had two or three chances (against Leicester) and scored two goals.
"For him it's really good, for everybody it's really good.
"But once again, Alvaro couldn't score his goals without the work of his teammates. We are a team and Alvaro scored at Leicester but on other days other players will score."
Negredo's performance had been heavily scrutinised as a result of his lack of goals but Karanka is unsure as to whether or not the former Sevilla and Manchester City striker was aware of any criticism - and he feels that may have been of benefit to the 31-year-old.
"I don't know," said Karanka when asked whether Negredo was aware of the criticism.
"I didn't speak to him about it and sometimes I don't know whether he understands.
"When I arrived here [from Real Madrid] it was good for me because I didn't understand anything, so that was good for me."