The Red Devils were gifted the victory by teenage goalkeeper Mile Svilar, who, rather comically, carried the ball over his own goal-line when he caught Marcus Rashford's deep free-kick.
The 18-year-old was making his Champions League debut and became the youngest goalkeeper in the competitions history, but it would be a night to forget after his horrendous mistake.
The error was the highlight of a lacklustre affair in the Portuguese capital. United dominated play in the second half, but their play was more about efficiency than being aesthetically pleasing.
Regardless, it was a good night for Jose Mourinho as his squad returns to England unscathed and well out in front at the top of Group A.