Southgate targeted the Portsmouth player after being handed the reins at the Riverside Stadium last summer, but had to wait until this week to make the breakthrough.
The 24-year-old was undergoing a medical on Teesside after the two clubs agreed a £5million fee, and Southgate said: "He is very comfortable in the role he plays, which we have not always had in that position. Although he is 24, he is a very experienced Premier League player, he is a quality footballer, he is a very good athlete, a very fit lad."
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