The 18-year-old winger was named in England's under-20 squad for the October international break and Tuchel felt it was appropriate for him to join the senior side for a training session.
Fellow United players Kobbie Mainoo, Mason Mount, Luke Shaw and Harry Maguire have all missed out once again.
Lacey trained with United’s first team when he was 16 and was named in the international under-20s group for the first time, after an impressive start to the 2025/26 campaign with the Red Devils' youth team.
He is seen as one of the players who can help mould the future at United, and the likes of Tuchel certainly think he has the quality to make it at the top after throwing him in with the first team.
England face Wales in a friendly on Thursday and Latvia in a World Cup qualifier next week.