Boro were the busiest Premier League club in July, making a total of eight new signings.
"We are not in a rush. We have the nucleus of the team so we just need to keep going," Karanka told BBC Tees.
"The better players we have, the more chance we have to stay in the Premier League."
Having narrowly missed promotion via the play-offs in 2015, Boro drew with promotion rivals Brighton on the final day of last season to earn promotion on goal difference and seal a return to the top flight after a seven-year absence.
The Teesside club have since been unbeaten throughout pre-season, but went a goal down after only 14 minutes of their 3-1 win over Aston Villa on Saturday - and Karanka is concerned about defensive frailties.
The 42-year old continued: "We need more [players], especially central defenders. That's the danger, where we need to improve.
"I don't want to put all the pressure on Dael Fry for example. He's brilliant, his future is more than brilliant and he's improving a lot every single day.
"But he needs to keep going forward doing the right steps and for that reason I think we need to bring [more players]."
Whilst Karanka wishes to bolster his defence, the Spaniard's priority is that any new signings fit into the team quickly.
"The main things for me is that [the new players] are settled in the team really well," he added.
"We are mixing players who two-and-a-half years ago were fighting to stay in the Championship with players who were playing in the Champions League two years ago, one year ago.
"But the spirit of the changing room is the same."
Middlesbrough will begin their Premier League campaign at home on Saturday, 13 August against Stoke City.
Boss Aitor Karanka says Middlesbrough are in no hurry to make further signings as they prepare to return to the Premier League.