City boss Kevin Keegan feels the player has not developed sufficiently enough to warrant a first-team place following his £4million transfer last summer.
"We felt he would be closer after what we had seen of him," he told the club's official website.
"When he got the injuries he should have been closer than he was but at that time he was not in any sort of form.
"A lot of things have happened to Matias. He came here not speaking one word of English and it was really only Paulo Wanchope or Juan Carlos Osario with whom he could communicate.
"His English is good now so he is over that one and he has got time on his side as well as he is only 21.
"I will have patience with him. But he has got some improving to do. But he is one of the best headers of a ball in the club and he is brave."