The Bayern Munich winger is a doubt for the competition as he continues to struggle with a back problem that kept him out of Sunday's 1-1 draw with Paraguay in Nice.
With Ribery's fitness in question ahead of the global showpiece, Grenier is under no illusions as to how difficult he would be to replace.
Both Grenier and Antoine Griezmann are candidates to fill the creative void should Ribery fail to make it, but the Lyon man feels Didier Deschamps could struggle to compensate for the absence of a player of Ribery's quality.
"(He) is a very important player for us," he told L'Equipe .
"He is the main player in our team and it is normal to talk about him. This is our best player, who can make the difference at any time and we really hope he will come back.
"(Griezmann has had) a great season, but replacing Franck is tricky."
Asked on his own likely contribution in Brazil, Grenier sought to play it down - insisting he is not expecting to a play a major role under Deschamps.
Instead, the 23-year-old is eager to simply make the most of any opportunities handed to him.
"I'm here to work and deliver what I am capable of. I'll take whatever I am given," he added.
"The only question I ask of myself is to make the most of the playing time they give me."
