Mattis joined the Silkmen from Chesterfield in January and marked his debut against AFC Wimbledon with a goal.
However, he suffered ankle ligament damage later in the same game and remains a minimum of three weeks away from a return to action.
"He's played at a higher level and he's got that experience - he's 30 so can bring a bit more to the team," Simpson said.
"He hadn't played for a bit so it was a decent display on his debut and he crowned it with a goal.
"Unfortunately he's got a really bad ankle and we're just not sure how long he's going to be out."
