Lauda, the Mercedes team chairman, told German television RTL: "He has been very spoiled in the past with many highs and victories, but that performance has gone.
"He has forgotten how to drive another car.
"With the (blown) diffuser cars, he had come up with a driving strategy that made him faster than everyone else.
"But now Ricciardo, who has never driven another Red Bull, only knows this one -- and he is taking it in his hands and driving it," added Lauda.
Lauda is not alone with his theory. Felipe Massa thinks Australian Ricciardo has arrived to be Vettel's teammate at precisely the right moment.
"The best time to change is when everything is changing," he told the Brazilian press.
"I'm sure it would have been harder for him if he had arrived last year, when Vettel was already used to the car for many years.
"I think that is helping Ricciardo, but he's also showing that he is a great driver with a lot of talent," said Massa.
Massa also thinks it will be a tough task for Vettel to get back to the top of the tree at Red Bull.
"When he had a team that was all working for him, for all the years when he was with Webber, life was much simpler, easier (for Vettel)," he said.
"But now he is under a little more pressure and that changes the situation -- the pressure effect," said Massa, who switched from Ferrari to Williams over the winter.
"I am not knocking Vettel's talent, the driver that he is, because everything he did deserves the recognition. But this is a situation that now he has to overcome," the Brazilian added.
"Without doubt it shows that Ricciardo is a good driver, fast and with the talent to be there. He has arrived at a great team and so far he shows he can do a great job," said Massa.