Amid McLaren's continuing performance crisis and rumblings of internal staff strife, Frenchman Boullier 'resigned' this week.
"It was a very difficult time for everyone at McLaren with a lot of pressure on our shoulders," team executive Brown told L'Equipe.
"In the end, he felt that McLaren needed to move forward. It was not a surprise. We have been under pressure and underperforming for too long."
When asked if Boullier was told to resign or he actually resigned, Brown answered: "I do not intend to go into detail about my exchanges with Eric. I don't think it's appropriate."
At the same time as Boullier's departure, McLaren announced that consultant Gil de Ferran will now step into a full-time sporting director role.
Brown said that Andrea Stella will be in charge on race weekends.
"The situation we are in did not occur in a day and will not be resolved in a day," Brown warned. "Today is the first step to return to the path to victory."
As for whether Boullier's existing role will be replaced, Brown said that he is unsure.
"This is the beginning of a process and I think we will examine it further in due course."
What is clear is that Fernando Alonso is probably happy with the changes, as he is close to Brown, his former Ferrari race engineer Stella, and Indy 500 champion de Ferran.
Brown admits that the Spaniard was "involved" in the restructuring.