The England international replaced the injured Matthijs de Ligt in the back three. He played for almost an hour before Victor Lindelof took his place.
The 32-year-old moved uneasily when he was substituted, but Amorim clarified that he was managing the defender after his injury comeback.
He said: "Just managing [him]. After 60 minutes, 55 minutes is a risk and we cannot risk any player in this moment."
De Ligt sustained an ankle injury against Nottingham Forest in midweek and there is no return date for the Dutchman at the current point.
Hence, Maguire's presence is hugely important. He is likely to start during the first leg of the Europa League quarter-final against Lyon.
Kobbie Mainoo did not make the squad after his return to training. The United starlet could be involved in Thursday's trip to France.