Red Bull boss Christian Horner has offered a diplomatic response to the possibility of Sebastian Vettel rejoining the team.

Silly season has kicked off early in 2019 with Max Verstappen and Mercedes fending off rumours of talks, while Vettel himself has had questions asked over his future amid ongoing disappointment at Ferrari.

An observation that is also often made is how frequently the German is seen at the Red Bull hospitality, but Horner says that is just part of the relationship between the driver and team who enjoyed so much success together.

"We have very nice memories of our time with Sebastian," Horner told Servus TV.

"He is part of the history of our team. He won four World Championship titles and many, many races together with us.

"Even though he's driving for Ferrari now, he still comes to see the mechanics more often. He knows most of the faces," the Briton noted.

"He's still a friend of the team, and he was very happy with us about the result at the Red Bull Ring. There is mutual respect and friendship."

Now, however, a report from Australia is suggesting Daniel Ricciardo could be considered as the Scuderia's 'Plan B' should Vettel either retire or rejoin Red Bull.

According to Auto Action, the Renault driver has a clause in his contract should Mercedes or Ferrari come calling.

If he was not available then Mercedes' Valtteri Bottas, who is out of contract after this season, would be the Italian team's 'Plan C'.

One move that is not on the cards, however, is Verstappen to Ferrari.

“No, certainly not and I can only confirm what Seb said,” team boss Mattia Binotto told Sky Sports.

“We’ve got two drivers, we’ve got contracts in place and we are very happy.

“Next season our line-up is fixed and there are no reasons why we should change. I think things are going well in that respect.”