The experienced centre-back came through the ranks at United and made almost 200 senior appearances under Sir Alex Ferguson, before departing in 2015.
After leaving Leicester City - where he won the FA Cup in 2021 - at the end of last season, the Northern Ireland international has been training with the Reds and signed a short-term contract in order to feature in pre-season.
Wearing the number 27 shirt, he was a solid presence at the heart of the defence alongside young Will Fish in the second half of Wednesday’s 1-0 friendly win over Lyon in Scotland.