Obi rejected pro terms at Arsenal after spending several years inside the academy ahead of leaving for United. The 17 year-old has since made three senior appearances for United.
"Obviously, when a player decides that the best pathway for him is to leave, there's not much you can do," said Arteta.
"Very unfortunate because we want to keep our players from the academy and to be very successful with the first team. But in that case, with Ayden (Heaven), we weren't able to do that.
"To persuade a player to stay a player has to be willing to be with us and commit to us and I don’t know specifically, both parties will feel very different about it.
"The reality is that he has decided to move and we have moved on from that."
Asked about Obi's qualities, he said: '‘Massive goal threat, an attitude and love to be inside the box and a capacity to score in many different ways. That was him."