Lawyers are “closely watching” what Mr Amaral will say in the show about the disappearance, which is due to air this month.
Kate and Gerry sued him for defamation after he penned a damning book pointing the finger of blame at them.
He claimed Madeleine, who vanished from Praia da Luz in 2007, died in an accident and her parents covered it up.
Mr Amaral, who lives in Lisbon, pocketed more than £350,000 from his book The Truth Of The Lie and a DVD spin-off.
The couple sued in the Portuguese courts, sparking an eight-year legal battle that still rumbles on.
An injunction against the book was issued in 2009 and the McCanns won the libel action six years later.
However, the ruling was overturned on appeal and that decision was upheld by Portugal’s Supreme Court in 2017.
The McCanns are now challenging him at the European Court of Human Rights after being told to pay £750,000 compensation.
They fear if they lose, Mr Amaral’s payout will wipe out a fund set up to finance the continuing search for Madeleine.
Mr Amaral, 59, is one of more than 30 interviewees who have taken part in the British-made Netflix documentary.
Because it has been produced in the UK, the McCanns could sue Mr Amaral in this country over any claims he makes.
The couple turned down the chance to see a preview of the documentary but their lawyers will watch it when it is aired.
A source said: “They have no desire to watch the show but they have been made aware Mr Amaral has taken part.
“Obviously what he says will be monitored in light of the previous false allegations he has made.”
Kate and Gerry, from Rothley, Leicestershire, said they still believe their daughter is alive because there is no evidence she has died.
Source:: Daily times