Davies, 26 was at the Lane on loan last season between mid September to the end of December; making seven (+1) appearance under Kevin Blackwell.
He initially started his career at the Riverside Stadium in October 2002 before moving to Southampton five years later in a £1,000,000 deal after loan spells with Derby County and Queens Park Rangers.
The 6ft 2inch defender joins Tony Mowbray’s side on loan until the end of the season – he has also featured for Preston North End and Walsall in loan spells.
United face Boro at the Lane on 9 April.