However, the 19-year-old has confirmed that the deal is only a temporary one.
"I'm a Middlesbrough lad and my mates all support the club," he said.
"They gave me some stick when I told them I was signing for Sunderland but, thankfully, they have calmed down a bit now.
"I enjoyed my debut, but it has always been my aim to make it with Boro, although I'm delighted Sunderland have given me this chance to play and I want to do well for them."