Scotland suffered a heartbreaking late defeat to Wales in their last qualifier to pile the pressure on Levein, a position not lost on Fletcher.
The Manchester United midfielder made his return to the international setup in the Wales match after a prolonged spell on the sidelines with a bowel disease.
He told Sky Sports News that the players will fight for Levein in Brussels tomorrow night.
Fletcher said: "We are desperate for the performance and the win for the manager because we believe in him as a team, as a group.
"We've seen the work he puts in, the way he treats the players, the attention to detail that goes into every game.
"The players are fully behind the manager. I don't think we could be better prepared for matches than we are. I honestly believe that."