Earlier in 2017, the 2009 world champion stood in for Fernando Alonso in Monaco, but Button indicated at Suzuka that he wants to return to another series full-time next year.
"I miss racing," said Button, present at the Japanese Grand Prix as a McLaren brand ambassador.
"I had sort of fallen out of love a little bit - I think maybe I left it a year too long in F1. But I want to go racing and have fun again."
Button said that he has "some sort" of contract with McLaren for 2018, but team boss Boullier admitted that the 37-year-old could actually leave his role with the team.
"In my view, he wants to spend next season in a racing series, and it is obvious that we cannot offer him that," said the Frenchman.
"But he is also an ambassador of the McLaren brand, and we would be happy to continue this cooperation."
However, Boullier admitted that Button's new racing role could clash with his ambassador duties, and therefore McLaren would need to seek a new official reserve driver.
Lando Norris is the leading candidate.
"Yes," Boullier admitted. "Let's see, because if he becomes F3 champion, which is quite possible, he will have enough points to receive the super licence."
McLaren are ninth in the constructors' standings ahead of tomorrow's race in Suzuka.