The former Arsenal captain rejoined the Catalan outfit last summer but has been left disappointed by his lack of match time as he faces competition from the likes of Xavi, Iniesta, Sergio Busquets and Alex Song.
The Euro 2012 winner has started all three of the club's La Liga matches so far this campaign but has failed to complete any of them, and the 25-year-old is not satisfied.
Fabregas 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 teammates 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 teammates or manager see it."
Fabregas's contract contains a £35m buy-back clause with Arsenal.