Maguire has reportedly been left in Manchester as he's dealing with a small muscular tear.
United have yet to confirm if that is indeed the case, although the England defender has revealed he'll miss the club's clash with Parisians due to injury.
"Disappointed to miss tomorrow's game through injury," the world's most expensive defender wrote on Instagram. "I'll be working hard to get back fit ASAP.
"The start of our #ChampionsLeague campaign #MUFC."