James Wilson opened the scoring but limped off after an hour as strikes by Tarique Fosu and Harry Cardwell threatened to end the Reds' season. Instead, James Weir, who replaced Wilson, grabbed his third goal in two games and, improbably, centre-back Paddy McNair arrived right on cue at the edge of the box to belt in an injury-time winner.
Wilson produced a shot on target within four minutes, although Andreas Pereira may have been a better option, and another break saw Saidy Janko fire wide of the far post when not maximising an opportunity after being picked out in lots of space by Joe Rothwell.
It was Rothwell who supplied the assist for the opener as his pass in the right-hand channel evaded the defence and Wilson's eyes lit up as he accelerated into the clear. Dan Lincoln was given no chance as the striker glanced towards the middle before clinically finding the net.
Nobody was in the six-yard box for a tap-in when the impressive Reece James sent over an inviting centre but the Royals had chances themselves in the first period. Charni Ekangamene halted Gozie Ugwu in full flight and the powerfully-built hitman later lashed just over Ben Amos' bar.
Aaron Tshibola, who played for United in the Aegon Future Cup two years ago, shot off target, as did Liam Kelly, with Martin Kuhl's youngsters finishing the half the stronger. The second 45 minutes started in similar fashion with Tshibola's effort simple for Amos but Jack Stacey should have hit the target when heading over a cross that bounced over James.