Britain's Jenson Button looks set to replace Fernando Alonso at McLaren for the Monaco Grand Prix in May.
Alonso will miss Monaco to race in the Indianapolis 500, with full support from McLaren and engine partner Honda.
McLaren executive director Zak Brown says two-time champion Alonso's replacement is "not in place".
But there is no other serious option than Button, 37, who is contracted to McLaren as a reserve driver and will race barring unexpected circumstances.
The 2009 world champion retired from Formula 1 at the end of last season and has spent the winter in California training for Ironman triathlons, his long-time passion.
He signed a contract with McLaren last autumn that committed him to replacing any race driver who was not able to take part in a grand prix this year.
As part of that contract, the team also has an option on signing him to race in 2018.
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Button tweeted a jokey reaction on Wednesday after the news of Alonso's Indy programme was announced, asking: "Why do I have so many missed calls?"
Meanwhile, Brown told a Bahrain news conference, held to discuss Alonso's Indy programme, that "conversations were ongoing" over the Spaniard's replacement.
"We have a few different options, we will state who that is when we know," he said.
"Eric [Boullier], who runs the F1 team, is ultimately responsible for making the recommendation as to what driver should go in the car and I think he will be here at the weekend so I can save questions for him, he is working on it.
"I wouldn't want to share the conversations he has had with whom."
Button's compatriot and former team-mate, Mercedes driver Lewis Hamilton, said: "I hope Jenson comes back and it will be great for the sport if Jenson comes in.
"Jenson is still one of the best drivers in the world and his calibre and experience make him the best choice."
Hamilton and Button drove together at McLaren between 2010 and 2012.
Are any other drivers in contention?
Button is the only serious alternative McLaren have. Alonso is one of the top three or four drivers in the world and they need an experienced replacement for a race where they have one of their best chances of the season to score decent points.
The McLaren chassis is quite strong, but the car is being let down by its Honda engine, which is said to be at least 100bhp down on the best in F1.
Monaco is one of the tracks on the calendar where engine power is least important in determining lap time. Alonso finished fifth there last year, when Honda also had a power deficit.
On top of that, there are very few available drivers with the required level of skill and expertise.
Mexican Esteban Gutierrez and Brazil's Felipe Nasr both raced last year and are potentially free, but are non-starters for a team such as McLaren needing to find a replacement for a two-time champion.
And Button's deal was struck with exactly this sort of situation in mind.