Despite starting all three Primera Division games so far this term, Fabregas remains far from happy playing a cameo role for his boyhood club, having returned to the Nou Camp last summer for £30million after eight years at Arsenal.
He told Radio Marca: "There's no such thing as a great substitute in the world of football.
"I couldn't tell you what makes a great substitute.
"I always wish my team-mates well and put a happy face on.
"If I have to take my unhappy face home then so be it, but I would never let my team-mates or manager see it."