Di Matteo is waiting for Marek Cech to recover full fitness before making Ipswich target Mattock available.
He told the WBA website: "Joe has trained, Marek has not trained yet, so Joe theoretically should be ahead of him. I would say Marek will hopefully be available again after the next international break.
"We were looking to increase the competition in the squad.
"If you want to be competitive the only way is to increase competition in the squad.
"I'm pretty happy. Nicky has come in and done a good job.
"When the time comes, and others are fit, I will think about that situation."