Red Bull is not yet shutting the door on Nico Hulkenberg.
It is believed Haas is in advanced negotiations with the Renault refugee, but reports indicate Hulkenberg is also interested in being Max Verstappen's 2020 teammate.
Red Bull has said that Alex Albon, who has replaced the struggling Pierre Gasly as of Spa, is only guaranteed the seat for the rest of 2019.
"He has nine races to prove himself," boss Christian Horner told Auto Motor und Sport. "Then we'll take stock."
Red Bull says all three of its drivers - Albon, Gasly and Daniil Kvyat - are in the running for the 2020 cockpit.
Horner said: "It is unlikely that we will hire a driver from outside of our squad."
But he also admitted: "We are watching the market".
"If we feel that none of our drivers can do the job, we will act."
Horner said Hulkenberg, 32, would be the "conservative" choice for a top team needing to maximise points for the constructors' world championship.
"It's always easy to be conservative, but sometimes you have to take risks," he said.
"Young drivers like Alex are a risk, but we showed in the past with Vettel and Verstappen that risk can pay off very well.
"Of course we are monitoring the situation externally, which is how we got Mark Webber. But we are mainly looking within our own talent programme," added Horner.
