Since moving to the German works team in 2013, Hamilton has won 11 championships in seven years, winning five drivers' titles and six constructors' titles in the turbo-hybrid era.
Alonso, who left Formula 1 at the end of 2018 and proceeded to win the WEC title and the 24 Hours of Le Mans this year, spoke on the Hamilton rumours at the FIA's gala in France.
"He has to consider all the situations and ingredients that will be in the mix and make the decision. I think it is very personal," the two-time champion told Channel 4.
Pressed on how he thinks the Brit would handle the situation, jumping ship from Mercedes to Ferrari, the former Ferrari star explained how difficult this would be.
"I think there will be some loyalty probably to put in place with Mercedes as well, for all the achievements they've had together.
"Also I don't know what Mercedes are thinking as well for 2021."
Like most of the drivers on the grid, Hamilton's contract runs until the end of 2020, giving him the freedom to jump teams when the regulations change in 2021, potentially giving a new team a head-start.