The teenager got to work with manager Erik ten Hag and the senior squad this week.
Lacey took to social media after the news was made public, as photos of him in training were doing the rounds among fans.
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"Been a tough 6 months good to be back out there," the talented teenager wrote on Instagram, while adding more photos from the session.
United may decide to involve Lacey in their squad for upcoming Premier League games.
The Red Devils take on Bournemouth at the weekend, before facing Coventry City in the FA Cup semi final at Wembley.