“The relationship is great,” said team boss Toto Wolff of Bottas.
“I know him since 2008, and he's not only driving for our team, but he's also my friend.
“He's always going to stay in the family, not only personally but for the team and Mercedes even though he's going be a fully-fledged Alfa driver.
“But he's been such a big part of the success of this team that he's not going to be leaving us, he's just in another garage.”
Wolff has previously acted as Bottas' manager, together with Didier Coton and two-time world champion Mika Hakkinen, having played a key role in the Finn's junior career.
Both entered F1 with Williams, Wolff buying a share of the team and joining the board of directors in 2009 before being named executive director of the squad in 2012.
Bottas meanwhile joined the team in 2013, the same year Wolff departed the operation to become executive director of the Mercedes F1 operation.
The Finn then joined Mercedes following the retirement of Nico Rosberg at the end of 2016, at which point Wolff stepped back from managing the then-27-year-old's affairs.
During Bottas' five years with the Anglo-German squad, it claimed the constructors' championship every year, and he personally claimed 10 race wins.
Following the Abu Dhabi Grand Prix earlier this month, he had a maiden outing with Alfa Romeo Sauber in the post-season test ahead of formally joining the squad alongside Guanyu Zhou for 2022.
Maybe this really is the last video… 😢 VB and Toto sign off on five fabulous years together. 💙 pic.twitter.com/mKAYV6dkW4