The Spanish veteran spent five seasons with the Scuderia but could only finish runner-up in three title battles with Red Bull’s Sebastian Vettel.
Alonso is confident that the Italian team has already put together a car capable of challenging for the title, and that Hamilton’s experience could push them over the line.
“It was a surprise,” he stated at Aston Martin’s 2024 car launch on Monday. “I will not lie, not because the change itself but from the outside it seemed like he was very linked to Mercedes and very loyal to Mercedes.
"Ferrari is a special team but it is more special when you win. Maybe Lewis can bring that extra fight for the championship because the car is there."
Hamilton's departure from Mercedes will create a vacancy alongside George Russell, while Verstappen's Red Bull team are also due to have a seat available with Sergio Perez's contract expiring at the end of the season.
Alonso's two-season deal with Aston Martin is up for renewal in 2025. It is understood the British team are keen to retain the double world champion's services.
"I am aware of my situation which is very unique," said Alonso, who won back-to-back world championships with Renault in 2004 and 2005.
"There are only three world champions on the grid and I am the only one available for 2025, so I am in a good position.
"On a move to Mercedes, there has been nothing at all. I know the driver market has started earlier this year, but this will not affect me in terms of preparing for the season.
"If I want to keep racing beyond this year, the first and only talk at the beginning will be Aston Martin. I trust this project and that will be my first priority.
"But if we cannot reach an agreement I know that I am attractive to other teams. I will not stay in Formula One just to have fun. I am not that kind of person and not that kind of driver. Let's see what the options are."
Alonso finished fourth in last year's drivers' championship as his Aston Martin transformed from also-rans to frontrunners.
Alonso scored six podiums in the opening eight rounds and might have ended his 10-year wait for victory had his team's strategy been more on point at the rain-hit Monaco Grand Prix.
The Silverstone-based team's form fell away in the second half of the year but Alonso continued to out-perform his machinery, taking further podiums in the Netherlands and Brazil.
— Aston Martin Aramco F1 Team (@AstonMartinF1) February 12, 2024
He ended the year 132 points and six places ahead of teammate Lance Stroll - son of the team's fashion billionaire owner Lawrence Stroll - and was voted third, behind Max Verstappen and Hamilton, by his fellow drivers and finished runner-up to Verstappen in a poll of the grid's team bosses.
Alonso, 43 in July, will this season become the first driver to take part in 400 grands prix and the evergreen Spaniard raised the prospect of competing deep into his forties.
He added: "A few years ago I would say 41 or 42 is the limit, but after last year I saw I was motivated, performing well, and I was thinking I could race a few more years.
"Now, this winter, I have exceeded expectations in terms of the physical tests, so if I am motivated and ready to commit, I can race until 48, 49 or maybe even 50."
Pre-season testing starts in Bahrain a week on Wednesday ahead of the first race, also in the Gulf Kingdom, on March 2.
Additional reporting by PA.