"It's a big blow, it's not what we had planned," Brunel said after the flair of Fijians dominated in a game where France got two first half tries but failed to score after the break.
"Unfortunately, it is the end of the autumn tests and the players are all returning to their clubs, leaving us to deal with this difficult impression. It's going to be a bit complicated.
"We won't be working again this week, although we will try to review what has happened and understand our shortcomings. We are well aware that this defeat serves as a lesson."
Brunel said he thought France had prepared well but admitted his side did not get into the game.
"We couldn't change the course of the game. They dominated us with their impact, in the rucks and we were late with our support play and we couldn't put in place what we had planned.
"It's hard to see a positive side but I want to believe this is not what we are capable of. We showed something much better against South Africa and Argentina. But we can't get away from the fact we have lost to Fiji,"
France suffered a last gasp 29-26 defeat to South Africa in Paris in the first of their three home tests this mont, but last weekend beat Argentina 28-13 in Lille.
"We felt we were making progress. We moved two steps forward this month but now we've gone back a step," Brunel added.
It was the first time in 10 meetings that Fiji had beaten France and was a rare success over a top tier nation for the Pacific islanders, who just a fortnight ago lost 54-17 at Murrayfield.
France have been drawn in Pool C at the World Cup in Japan, where they will play Argentina first, then the United States, Tonga and England.
(Reporting by Mark Gleeson; Editing by John O'Brien)
