
One source told Reuters there could be an announcement by the end of the week, with reports in Spain and Italy saying talks between McLaren's Sainz and Ferrari were at an advanced stage.
"I see a scenario which is [Carlos] Sainz to Ferrari, [Daniel] Ricciardo to McLaren and then that leaves a seat at Renault that probably would be [Fernando] Alonso", he said, as he threw out a prediction on Sky Sports.
Sainz, who made his F1 debut with the Italy-based Red Bull-owned Toro Rosso team, is in Spain while Ricciardo went to his family's farm near Perth after the Australian Grand Prix was cancelled in March.
The 22-year-old was engaged in a power struggle with Vettel last season as he continuously out-performed the German.
He is now the favourite for one of the most coveted seats in motorsport.
In a question-and-answer video segment that was posted on Instagram, the Dutchman, whose contract runs to the end of 2023, said: "I guess also very soon we'll find out who the replacement is going to be. I'm at Red Bull".
Mercedes boss Toto Wolff says the Silver Arrows outfit is committed to keeping its driver line-up intact for 2021 but must also take into consideration Sebastian Vettel's availability.
Where Carlos Sainz finished in the drivers' championship previous year.
Wolff is therefore aware that he and Mercedes can't simply ignore the news.
"When looking to the future, our first loyalty lies with the current Mercedes drivers".
Signing Sainz will end any lingering speculation about six-time champion Lewis Hamilton leaving Mercedes for Ferrari.
Ferrari had many options when moving forward with finding a long-term successor for Vettel.
"Sebastian is a great driver, a great personality and an enrichment for every Formula 1 team", Wolff told German media outlet DPA.
Always a quick driver, Sainz has done enough in his five years in F1 to show that he has outgrown the midfield.
Daniel Ricciardo at the Canadian Grand Prix past year.