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.

But the charge was reduced to a misdemeanour, and he received a one-year suspended prison sentence, escaping a longer term.
He was acquitted of a second charge of breach of trust.
The "Lagarde list" of HSBC bank account holders caused outrage in Greece as it faced its economic crisis.
Greek governments have been accused of not fully investigating the list, which was given to Athens in 2010 by the then French Finance Minister Christine Lagarde.
It is an extract of a list of account holders at a Geneva branch of HSBC leaked by former bank employee Herve Falciani.
But the Pasok government took no action after receiving it and later claimed to have lost it.
Papaconstantinou, who was finance minister at the time, denied any wrongdoing.
The court in Athens found him guilty of doctoring a document, but the charge was reduced from a felony to a misdemeanour.
Papaconstantinou was expelled from Pasok in 2012 after the allegations emerged.
The country's new leftist-led government of Prime Minister Alexis Tsipras has promised to crack down on tax evasion.

The Faraday Lecture Block dates back to the 1960s and the university says its "tired and monotonous design" means it has become a bit of an eyesore.
As well as new multi-coloured external glass cladding, the building will have a new entrance hall, extended second floor and improved access.
Work on the 11-month project is due to start in January.
The university said the new look would add vibrancy to the building and the mall area.
The majority of construction will take place during the Easter holidays and summer months.
A spokeswoman said: "It'll create a good impression and reflect the standards that Swansea holds and a professional atmosphere that fits in with the degrees that are taught in that building".
Boss Aitor Karanka says Middlesbrough are in no hurry to make further signings as they prepare to return to the Premier League.
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Former Greek Finance Minister George Papaconstantinou has been found guilty of removing relatives' names from a list of potential Greek tax evaders.
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One of the main buildings on Swansea University's Singleton Park campus is to undergo a £2.75m makeover.