The first half left a lot to be desired from both sides, neither making a solid attempt on goal, and the game looked like it may be ebbing away from the Reds when Matej Vydra sent Plzen ahead a few minutes after the interval.
United boss Ruben Amorim noticed the need for a change, making substitutions either side of the hour mark and introducing Rasmus Hojlund, Antony and Mason Mount to the field.
It was the first of those, Hojlund, who would put his stamp on the game, feeding off some strong wing play from Amad for his first, before notching his second, and the winner, after clever work by Bruno Fernandes at a free-kick.