He has not been at his best during the start to the current campaign and also had to deal with a family bereavement which meant he was excluded from the recent game with Tottenham.
But there were encouraging signs when he appeared as a second-half substitute against Birmingham on Saturday and helped Albion recover from being 1-0 down to win 3-1.
Di Matteo said: "Graham is a very important player for us and we have a lot of faith in him.
"We are waiting for him to hit the form that he was in last season.
"We know his ability. We saw last season how good he was. We see it every day in training.
"We just need to be patient with him and give him the time to reach that form - and he will.
"There is no doubt about it and he did well against Birmingham."