According to the Manchester Evening News, Manchester United have submitted their 25-player Champions League A list to UEFA. Mainoo and Yoro are not included, but they can still play in the competition.

The Champions League will return to Old Trafford next week. Manchester United submitted a full 25-player list, with Mainoo and Yoro among the notable absentees.
Champions League clubs this season must submit their A lists to UEFA before Wednesday. Once the league phase begins, the lists will be locked; clubs that advance to the knockout stages can replace up to three players after the league phase ends.
Mainoo and Yoro will be included on the B list submitted to UEFA before each match. A player qualifies for the B list if they were born on or after January 1, 2005, and have been continuously registered with the club for at least two years after turning 15, or for three years including any loan period. Mainoo and Yoro both meet the requirements, so they do not need to occupy A-list places.
Haven and Tynan-Thompson have been included in the A list because they have not been with Manchester United long enough and do not yet meet the B-list eligibility requirements.