Blues boss Di Matteo said: "Strikers are instinctive, it is difficult to understand what goes through their minds.
"But I have shown in the past with my rotation of the team that I have done that with everybody.
"Fernando has always cared but he had a difficult time.
"This season it's different. He is playing well, looking good and I would expect that to continue. I have no concerns about Fernando. He always works hard for the team and, whether he scores or not, he always contributes. He is very useful for attacking combinations.
"We can't just rely on one player to score anyway. We want to share out the goalscoring responsibilities and Fernando has been training well. Maybe Tuesday was not his best game but it's not a question of him being happy or not. He is in a good state of mind.
"We now need to move forward as we are talking too much about the past. Yes, Fernando had a difficult season last term. He made that clear. But rather than mulling over the past, let us look to the present and future.
"The facts this season are different. Fernando has scored four goals in the league and one in the Capital One Cup so far.
"If you see the games we have played, it's one in two."