Striker Chido Obi will miss Manchester United's game against Real Sociedad because he's ineligible to play in the UEFA Europa League.

The 17-year-old impressed after coming on as a substitute during United's FA Cup defeat to Fulham on Sunday.

It was just his third senior appearance after moving from Arsenal in October.

His performance against Fulham prompted questions to head coach Ruben Amorim about whether he is close to making his first start in place of under-fire forward Rasmus Højlund, who hasn't scored since December.

However, Chido Obi won't be able to face Real Sociedad in the Europa League round-of-16 first leg on Thursday because he is ineligible to play in the competition.

United added January signings Patrick Dorgu and Ayden Heaven to their Europa League squad ahead of the knockout rounds, which begin with the trip to Spain this week.

Chido Obi was overlooked, a source has told ESPN, because he was not considered a first-team player at the time.

A number of injuries to senior players, including Amad Diallo and Kobbie Mainoo, have seen the teenager fast-tracked into Amorim's squad.

Chido Obi does not qualify for a place on the B list -- usually reserved for young players -- because, in line with UEFA's rules, players must have been at the club for a minimum of two years.

Being without the Denmark youth international is another blow for Amorim, who only named seven substitutes against Fulham instead of the permitted nine.

United are already sweating on the fitness of Harry Maguire after he had to be substituted against Fulham with a muscle problem.

The England defender is due to undergo a scan on Monday to assess the severity of the problem.