However, Boro boss Southgate has confirmed that O'Neil has committed his future to the club.
"I am delighted. He has come back a lot more settled and he is determined to try to give it a go," Southgate has said.
"He feels after the second half of last season, he owes us a little bit, and it is fantastic to know that.
"I am delighted. He was superb up to Christmas for us and then because he was unsettled and doing a lot of travelling, that affected his performances.
"But I am thrilled that he has come back and wants to play a big part with us because we know the quality of his play."