Adam Lawrence’s side fell behind inside three minutes when Anis Soubier’s effort from the edge of the area was mishandled by goalkeeper Cameron Byrne-Hughes. Monaco doubled their advantage in the 13th minute, Sirak Bein rising above the United defence to glance home Aymen Assab’s corner.
United had plenty of possession during the first half but struggled to find a way through Monaco’s physically imposing defence. Amir Ibragimov and Jim Thwaites both saw efforts blocked, while Harry Amass and Tynan Thompson made promising progress down the left without finding a decisive final ball.
Monaco continued to threaten after the interval, with Dan Armer blocking an early Soubier attempt. United nevertheless reduced the deficit eight minutes into the second half when Thwaites delivered a corner from the left and goalkeeper Jules Stawiecki failed to keep the ball out under pressure from Reece Munro.
The hosts briefly looked capable of building on that goal, but Byrne-Hughes had to make an important save to deny Nahel Haddani before Monaco restored their two-goal advantage. Substitute Yannick Dodo had only recently entered the field when he collected Najd Ajroud’s pass and finished emphatically in the 63rd minute.
United continued to press and created further opportunities through Ibragimov, Thompson and Bendito Mantato. Ashton Missin also went close after coming on, but the visitors held firm until stoppage time.
Kana-Biyik, introduced in the 81st minute, eventually found the net in the fourth additional minute, glancing home from another Thwaites corner. It was too late to find an equaliser, however, and Monaco left Leigh with the points.
Match details:
Manchester United: Byrne-Hughes; Kamason, Munro, Armer, Amass; T. Fletcher, Thwaites, J. Fletcher (c); Mantato (Missin 75), Ibragimov (Kana-Biyik 81), Thompson.
Subs not used: Heath, Helafu, McCormack, Leon, Orozco.
Goals: Stawiecki (own goal) 53, Kana-Biyik 90+4.
Booked: J. Fletcher 78.
Monaco: Stawiecki; Gomes, Wami, Bein, Mokabakila (Leunga Leung 84); Nibombe (c), Benzahra (Drame 84), Ajroud; Soubier (El Ouaroudi 53), Haddani (Dodo 62), Assab (Belhokhtar 62).
Substitutes: Tiamuna, Dzabatou.
Goals: Byrne-Hughes (own goal) 3, Bein 13, Dodo 63.
Manchester United will return to the competition in early December, when they host Juventus. Their immediate focus switches back to Premier League 2, with a trip to Burnley next on the schedule.