Earlier in the summer Portsmouth seemed to be interested in taking the Nigerian back to Fratton Park but their offer was unacceptable to Boro, who are believed to value the player at £12m.
The Teessiders are short of attacking options with the deparure of Mark Viduka and Malcolm Christie over the summer.
On paper the only experienced out-and-out strikers at the club, aside from Yakubu, are Jeremie Aliadiere and Dong-Gook Lee.