It is strongly rumoured that Sebastian Vettel's long-time teammate Mark Webber, 36, will not stay at the world champion team for an eighth consecutive season in 2014.
Reportedly at the front of the grid to replace him are Lotus' Kimi Raikkonen and Toro Rosso's Daniel Ricciardo, although Jean-Eric Vergne also cannot be ruled out.
"I expect from both of the (Toro Rosso) drivers a very competitive second half of the season," said Toro Rosso boss Franz Tost, referring to the similarly 23-year-old Ricciardo and Vergne.
"Then we will see how the future will be."
Red Bull team boss Christian Horner also said he is keeping an eye on the young duo.
"They're both young, talented drivers, both developing very well. It's good to see," he said in Monaco on Thursday.
"For sure they're two guys we have a watchful eye over."
But Berger, who in 2007 oversaw Vettel's ultra-successful transition from Toro Rosso to Red Bull, thinks it could be too soon for Australian Ricciardo or Frenchman Vergne.
"Red Bull needs two absolutely winning drivers," he told APA news agency.
"Whether one of both of these boys have it, we will see in the second half of the season. But now it's still too early to judge.
"I think they may need another year of development at Toro Rosso."
Asked about the Raikkonen speculation, fellow former McLaren and Ferrari driver Berger answered: "Kimi would definitely be a very good option (for Red Bull)."