But two-time championship winner Alonso is about to enter the final year of his three-year deal with McLaren, meaning Mercedes may have to look elsewhere.
"You have to consider Fernando," Mercedes boss Toto Wolff told Sky Sports' F1 Special: Nico The Brave show.
"He is a driver I respect a lot. He combines talent, speed and experience. It's all there.
"But he is in a contract with McLaren-Honda at the moment and we just need to weigh all the other options up."
Mercedes face a race against time to bring in a new driver alongside Lewis Hamilton, with pre-season testing due to begin in February.
Rosberg made his surprise announcement last week, just five days after sealing the drivers' championship.
Former Mercedes test driver Pascal Wehrlein, 22, remains the most realistic option for Wolff, who insists both his drivers will share top billing.
"We will maintain the system we have," he added.
"Both drivers will have equal status and equal opportunity to the best of our abilities. We owe it to the fans. You can't have a pecking order."