After wasting a one-on-one chance, the substitute broke the deadlock from 10 yards as improving Wycombe earned a sixth away win of their campaign.
It was a cagey match with both sides exercising a cautious approach but it finally came alive when Rovers substitute broke the deadlock.
The substitute broke into the box and drilled an unstoppable shot past Tomas Vaclik to equalise on the night and preserve the Blues' one-goal advantage from the first leg.
The proliferation of substitutes broke up the tempo, although Morrison's pace and directness continued to cause the Danish defence problems and, in injury time Colin Healy and David Connolly combined to leave him a chance which he side-footed in with the composure of a veteran.
With Adu returning to the role of second-half substitute, United broke a five-game streak without a victory Saturday night.
Chris Wood, another home substitute, almost broke the deadlock late on but his low 20-yard attempt struck the base of a post.
Resolute defending kept City in the tie up until the second minute of stoppage time when the substitute Dani Sanchez broke the deadlock.
Then, in the 92nd minute, the Swansea substitute Marvin Emnes broke with pace and while Fabricio Coloccini resisted the temptation to tackle in his own penalty area, Cheick Tioté dived in and clattered the striker.
But substitute Cuvelier broke Pools hearts with his close-range winner.
But substitute Steven Naismith broke Villa's resistance with a 16-yard shot.
Substitute Ben Williamson broke Forest Green hearts as his late winner saw Port Vale edge home at The New Lawn.
