That deal, marking the return of the historic Alfa Romeo name to Formula One after a more than 30-year absence, was announced earlier in the week.
Ferrari was known to be keen on placing both its outstanding junior drivers, Charles Leclerc and Antonio Giovinazzi, at Sauber for 2018 as part of an expanded technical partnership with the team.
The lineup leaves Pascal Wehrlein, Ericsson's German team mate, facing an uncertain future with former champions Williams the only remaining team with a vacancy but Poland's Robert Kubica favourite to fill it.
Giovinazzi's chances seemed to have been raised by news of the Alfa Romeo deal, with speculation Ferrari president Sergio Marchionne was keen to make it a fully-fledged junior team.More news: Gunmen attack agriculture institute in Pakistan, killing 12
The vehicle is predominantly white with red accents, and large Alfa Romeo logos after the Swiss team confirmed a deal with the manufacturer. Instead, the Italian will be Sauber's third driver, effectively meaning he is in a reserve role.
Sauber finished last in the constructors' championship for the first time in their history in 2017.
Leclerc had always been tipped for one of the Sauber seats after storming to the Formula 2 title, finishing 72 points ahead of Artem Markelov. Ferrari celebrated the 70th anniversary of his first auto this year.
"It was a great experience to drive a few FP1 sessions and tyre tests for the Sauber F1 Team in 2017, during which the team welcomed me straight away", said Leclerc.