The Jimmy Murphy Young Player of the Year scored twice in the FA Youth Cup final victory over Nottingham Forest at Old Trafford and has enjoyed an excellent season for the Under-18s.
After making his senior debut against Chelsea and being involved in the first-team squad, it was anticipated he could feature at some stage in south London this weekend.
"Unfortunately, Alejandro was ill in the last couple of days," revealed interim boss Ralf Rangnick at his press conference to preview the Eagles clash. "He was training with us until midweek last week but then he had to drop out because he was ill and I think today was the first training session that he was back again.