After just a single season alongside then-rookie Hamilton in 2007, Spaniard Alonso left the British team.
"So if Hamilton is also evaluating that option, I don't see why anyone should say it (Hamilton leaving) is not possible.
"McLaren is a great team, but they won the world championship in 2008 only at the last corner of the last race. If we remove that title, you have to go back a long way to find another one," added Alonso.
Alonso and Hamilton clashed infamously as teammates in 2007, but it is clear the pair now have a great respect for one another.
Alonso confirmed: "I have said he is the only one capable of winning without having the best car. The others win if the car is good, and when it is not (good), they do not (win)."