The 44-year-old former Boro midfielder, who joined the League Two to work with Colin Cooper last May, handed his notice in on Monday afternoon.
Hignett played at Ayresome Park and the Riverside Stadiym between 1992 and 1998 and is set to become Aitor Karanka's assistant manager, starting work on Tuesday.
- Sports & Recreation
- Colin Cooper
- Aitor Karanka