The on-loan Bayern Munich midfielder has been pilloried by pundits over his form this season.
But Fabianski says: “He is still very young, people forget that.
“I can honestly say, from my own experience, when I first signed for Arsenal at 22, even though I wasn't a starting player, I could see the league is so different to any other.
“For any player it is always going to be a struggle to start to get used to the pace and the quality and it is the same for Renato. He is a young boy and you cannot forget that.
“I don't know how people portray him from the outside but I promise he is working very hard and wants to be a big part of this team. He wants to get himself going and we have to believe in the kid because he has big potential."
He added: "I'm pretty sure everybody will be very happy with him in the near future because in the squad we all know and believe Renato has a lot of quality and will show it. I think he will be very useful for us but you have to remember he is still a kid."