Former Ferrari driver - now Toro Rosso advisor - Gerhard Berger believes neither of Toro Rosso's current drivers are ready for the move to the main Red Bull team.
With speculation mounting claiming current Red Bull driver Mark Webber will leave the team at the end of this season, the pressure is on both Daniel Ricciardo and Jean-Eric Vergne to show the Red Bull bosses they are worthy of the seat alongside Sebastian Vettel.
So far the Toro Rosso driver impressing most is Ricciardo - the young Australian has qualified in the top 10 and scored good points this season and is on the front row as it were with Kimi Raikkonen in terms of a replacement for Webber.
Toro Rosso team boss Franz Tost has also been impressed but expects a big battle between his two drivers for the rest of the season.
"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 is also keeping an eye on the pair.
"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."
Despite the praise of Horner, advisor Berger, who oversaw Vettel's rise in 2007/08 at Toro Rosso, is unconvinced.
"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."
Berger instead believes a move for Kimi Raikkonen would be a better man for the main Red Bull team.
"Kimi would definitely be a very good option," he added.
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