Blackburn Rovers won the semi-final behind closed doors at Carrington by virtue of Osayamen Osawe's goal against the run of play in the 59th minute, when the striker beat Reds goalkeeper Conor Devlin to a left-flank delivery from Michael Potts.
Nick Ajose came closest to scoring for United in the first half, smashing the underside of the crossbar after bringing down Gabriel Obertan's right-wing cross at the far post. Robbie Brady also hit the woodwork - the angle of post and bar - with a ferocious shot when United were chasing an equaliser in the second period.
Devlin's opposite number Matthew Unwin thwarted Josh King on a couple of occasions before the break and was also forced into making good saves from Ajose and Bebe as the well-drilled visitors held on to their slender lead. Rovers had two decent chances to double their advantage but these were spurned by Tom Hitchcock and Jamie MacLaren, despite their good openings presented by United chasing the game.
United: Devlin; Vermijl, Wootton, Gill, Brady; Ajose, Petrucci, Norwood, Bebe; King, Obertan. Unused subs: Ekangamene, Coll, Rafael Leao.