Medical staff responded after fans and stewards waved to draw their attention to a medical emergency at the opposite end of the pitch during a break in play.
The game was halted for a few minutes and eventually the fan was taken a way, allowing play to resume.
Minutes later, an announcement was made over the tannoy system in the stadium to clarify that the fan is awake and well.
The incident came just as United defender Jonny Evans pulled up with an injury less than 15 minutes into the clash and had to be taken off, with Raphael Varane taking his place.
United were 1-0 up in the Champions League game after three minutes, with Rasmus Hojlund getting in at the back post to meet a low cross from Scott McTominay and tap in from close range.
Then, in the 28th minute, United went 2-0 up when Alejandro Garnacho's shot was saved and rolled into Hojlund's path and he, again, tapped in from close range.
