Dorrans has returned to training after time off for a family bereavement.
"He's an important player for us," Di Matteo told the WBA website. "He hasn't quite hit the form of last season yet.
"But he will reach it sooner or later and when he does he's going to be a vital player for us.
"He knows himself that he hasn't quite reached the form of last season.
"It's only normal."