"I know him very well, since he was 15 years old, so I know him for a long time. He is a top-class talent," Wenger told arsenal.com.
"He was here as a young boy but we cannot get players in here if they are not internationals. He had no work permit and was too young. We tried to wait until when he was in Belgium, we tried to get him a European passport.
"He was not patient enough and left for Metalurg Donetsk, that is the club where he went.
"He has made a big impact - he is a top class player. He came out of a frustrating season at Barcelona because he didn't play last season at all but the year before he played and was very influential.
"Last year he lost his position for Busquets and in midfield, he is the kind of player who needs to play, Yaya - we know him very well."