Here, Press Association Sport takes a closer look at the unfamiliar names who manager Jose Mourinho - with one eye on Wednesday's Europa League final against Ajax - has confirmed will be involved against Sam Allardyce's Eagles.
Made his United debut as a late substitute in the FA Cup win against Wigan in January, having been recalled from a loan spell at Portuguese club Belenenses. Tipped by Mourinho to be the "best Portuguese goalkeeper of the next generation".
Manchester-born former England youth international who played as a winger for much of his youth career. Has more recently converted to an attack-minded left-back and has impressed for United under-23s this term. Was an unused substitute in recent games at Tottenham and Southampton.
Has operated mainly in midfield for United reserves this season, but can do a job up front too at 6ft 4in. Made his Premier League debut as a substitute at Arsenal earlier this month. Born in Lancaster but recently told the club's website he feels " probably more Scottish than English".
Endured an injury-plagued 2015/16 campaign but has enjoyed a much more productive season this term. A skilful midfielder with an eye for goal, Harrop scored a hat-trick for the reserves against Tottenham - in front of a watching Mourinho - at Old Trafford earlier this week.
Former England schoolboys and youth international has had his share of injury woes but has shown himself capable of playing in midfield and in a more advanced role for United's academy side. Impressed for the under-23s as a second-half substitute in front of Mourinho on Monday.