Following the news from Honda that it is to withdraw from F1 at the end of 2021, Red Bull - and its sister team AlphaTauri - is left with few options.
Having been rebuffed by both Mercedes and Ferrari previously, it is unlikely that either would be willing to relent if asked again. Alternatively there is the possibility of buying Honda's operation or re-partnering with Renault, the French manufacturer obliged to provide its engines under the current rules.
Speaking to Red Bull's own TV station, Servus, Christian Horner admitted that a decision on its new engine partner must be confirmed in the coming months.
"We already need to think about the incorporation of the engine," he said. "Really, by the end of this year we need that clarity, we need that clarification, so of course we have to consider all options, all possibilities.
"Ultimately it will be Mr. Mateschitz's decision what he wants to do," he added, "but of course it is important for us to have enough power to challenge Mercedes in the future years.
"I understand why people assume that we will talk to Renault," he admitted. "Since the separation, Renault has changed. The new board brings a lot of fresh wind and some changes. Things are moving forward.
"The cost of getting a new manufacturer on board under the current regulations is simply far too high," he added, when asked about the prospect of a new manufacturer entering the sport. "So there will be no new manufacturer until a new engine, possibly in 2026, comes on the market. The costs for development are enormously high.
"The FIA and Liberty have to get a grip on this. They have done a good job on the chassis. Now we need homologated engines and we also need budget caps for the power units."
Asked about speculation since Honda's announcement that Max Verstappen has an exit-clause in his contract in the event of the team losing its works entgine partner, Horner said: "No clause in the contract exists like that.
"Of course contracts are private between the driver and the team," he added, "but there is no engine-related clause in Max's agreement.
"He is competitive," he continued, "he feels very comfortable in the team, he has been a big believer in the Honda programme, and I think he also sees that Honda has brought forward the 2022 engine into 2021, which is really encouraging. He's excited about that and for him 2022 is still a long way away."