The Austrian insists that the decision might not be made until late January.
Lauda made his comments during a conversation with Red Bull motorsport supremo Helmut Marko on the Servus TV channel, during which the old friends and adversaries teased each other.
"We had hoped for a faster solution, but the discussions prove to be difficult," admitted Lauda.
"There are two scenarios – we take a young gun like Wehrlein who could turn out to be a Verstappen, or he couldn't. Or we take an experienced guy.
"But we are still making up our minds. That's a huge discussion, really. It can take until the end of January until we know which route we are going to take."
Marko was keen to wind up Lauda about the apparent lack of faith in Wehrlein: “You have a young driver programme and you have the highly-rated Wehrlein.
"But according to our information it won't be Wehrlein in the car. This means you don't trust your own junior programme.”
Lauda replied: "I have to correct that one. The young driver programme is a junior driver programme, as you say. It's there for young drivers to get experience.
"Of course we can discuss about Wehrlein being ready or not. But I want to remind you of Max Verstappen first. He was parked over at Toro Rosso until the Russian [Daniil Kvyat] didn't drive properly anymore. Only then you moved him across [to Red Bull Racing].”
“Okay, Wehrlein is very young, but no risk, no fun,” said Marko. “So you take Bottas, but you have to give Williams a two-digit discount on the engine deal.
"You do have a budget of €22m, however, as this is what Rosberg would have cost you. But you also have to buy a seat for Wehrlein at a team which is running an engine from a competitor [Sauber]."
Lauda also revealed that if Bottas joins Mercedes then Toto Wolff will have to relinquish his management stake in the Finn, a result of an agreement made when Wolff first joined the team.
"Bottas has a contract with Didier Coton, Mika Hakkinen and Toto Wolff," continued Marko. "This means your team boss is involved in the management of the driver you hire. How's that about compliance?”
“Easy,” said Lauda. “It's been set up before Toto became CEO of the race team. If Bottas were to come to us Wolff cannot be his manager anymore."
Additional reporting by Stefan Ehlen