The Spanish title holders meet Celtic in the Champions League on Tuesday night but will be without the Argentine who sustained a thigh injury on the weekend.
Fabregas says it is vital that Barca unite without Messi in what is a major test of character for the Catalan club following last season’s defeat at Celtic Park in the group stage of Europe’s premier club competition.
"It is a test because we always want him in our team," Fabregas said. "Obviously Lionel is the best player in the world, I don't even have to say that and you obviously will miss a player of his talent and dedication.
"We rely on him a lot. He is the best player, the one that scores more goals, the one that does the offensive tasks. He is always there for us, he gives assists, he is the best.
"When he is not there we have to adapt the situation and players like Neymar, myself if I play, Alexis Sanchez, Cristian Tello, Pedro Rodriguez; we have to step up and do the job that a striker is made for which is scoring goals and giving assists.
"I think Tuesday is a test in that way."
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