Manager David Moyes told the official Everton website: "Obviously we would like to keep him. We have a deal with Borussia Dortmund to the end of the season so there's not an absolute desperation to do that.
"We are quite comfortable with it at the moment. The fee is agreed."
"I think he's enjoying himself," added Moyes. "He feels wanted here. He's comfortable with the surroundings.
"It's easier that he has other African players here as well. He would tell you that he's really enjoying his spell here. Does it surprise me that he's doing well? He was very good at Ajax.
"What worried me was that he went to Borussia Dortmund and it didn't happen for him. His stature sometimes gave me the feeling that there was no strength there."