Fernando Alonso would be top of the list for Mercedes if Formula One champion Lewis Hamilton fails to agree a new deal with the team beyond this year, according to motorsport director Toto Wolff.
Hamilton, who took his second championship in 2014 after winning 11 races, has yet to put pen to paper on a new contract despite both sides talking about their eagerness to stay together.
"There's no hurry, we'll discuss it during the year," Wolff told Italy's Gazetta dello Sport newspaper in an interview from his vacation in the Austrian ski resort of Kitzbuehel when asked about the talks.
"I'm an optimist. The priority is to continue with these drivers (Hamilton and Nico Rosberg). Should it not be possible, then Alonso represents the first alternative, then there's (Finland's Valtteri) Bottas."
McLaren said last month that Spaniard Alonso, a double world champion, had signed a multi-year deal in joining them from Ferrari.
However, Alonso made little secret of the fact that he was interested in the Mercedes seat and is likely to have exit clauses if McLaren, who are starting a new partnership with Honda after years with Mercedes, under-perform.
Wolff has emphasised repeatedly that Mercedes want Hamilton to stay long-term and his words could be seen also as a means of putting pressure on the Briton by suggesting the team have alternatives.
The Austrian paid tribute also to Alonso's abilities, saying he was "dangerous (as a rival) in any car.
"If he has a car that can finish sixth, he will take it to third place," said Wolff.
He dismissed Honda's poor showing in their first post-season test with the new engine and said it was an electronics issue, rather than anything more serious linked to the power unit, of a kind that all teams had experienced.
The Austrian also tipped Mercedes-powered Williams, the team he was with before Mercedes and which employs his wife Susie as a test driver as well as Bottas, as being a main rival in 2015 after they finished third overall last year.
"I put Williams at the top of the list," he said. "But I also fear a lot Red Bull. They have won four titles in a row and will use every resource in order to fight again for the title."