Di Matteo insists Scharner, like all his players, will have to earn the right to a regular starting place in his Barclays Premier League XI.
But he reiterated he views the former Wigan ace's future in the team as a midfielder.
"Nobody is guaranteed a place but I said to him I was recruiting him as a midfield player," he told the WBA website.
"We know that he can play as a centre-half as well but primarily he has been signed as a midfield player.
"It was hard work to get him.
"He has been a free agent since the end of last season and has had lots of interest from other clubs.
"Eventually we managed to attract him."