The Red Devils committed an error in the 2-1 loss, with goalkeeper Andre Onana passing straight to a Plzen player.
United did improve in the game, with substitute Rasmus Hojlund scoring twice in a 2-1 win.
Plzen manager Miroslav Koubek stated post-game: "Yes that was our tactic.
"I told them to shoot from every viable opportunity - maybe I'm disappointed with the very last shot - that could have been resolved better.
"But the player decided to shoot and it is what it is. We are disappointed, we deserved at least a point. I am proud of the team. The guys gave everything and made it very difficult for the opponent.
"The opponent certainly values the win a lot, we value our performance, it was a beautiful match. It wasn’t like a world-class opponent was pouring four or five goals on us, we resisted heroically.”