Craig Hignett says it was an extremely difficult decision to leave Hartlepool United and to head coach Aitor Karanka.
The 44-year-old gave up a youth coaching role with Boro to at Pools last May.
"I left Hartlepool with a heavy heart because there are a lot of good people over there," he told BBC Look North.
"It came out of the blue, but the step up from League Two to the Championship was too good for me to turn down."
Hignett enjoyed a 19-year playing career, starting his professional journey with Crewe Alexandra after being released by Liverpool as an 18-year-old.
In 1992, he signed for Middlesbrough and went on to score 48 times in 194 appearances for the club, as well as playing in the 1997 League and FA Cup finals.
"I've made friends for life at Hartlepool and it was hard to leave but Boro is my club," Hignett explained.
"I left on fantastic terms and they understand the opportunity I've been given."
The former midfielder has returned to the Riverside with the Teessiders in the midst of an alarming goal drought that has seen them slide down to 14th in the Championship table.
Boro were ninth, and only five points off the play-off places when they beat Charlton but they have failed to score in seven matches since.
"Hopefully, I can help the boys with that," Hignett added.
"I'm tremendously excited. The more time I can spend with Aitor, the more he'll understand me, and the more I'll be able to help."