The Reds are due to fly to Spain on Wednesday for the quarter-final first leg and Elanga is the latest youngster in contention for a place as Ole Gunnar Solskjaer continues to show his faith in youth.
Clubs are allowed to add homegrown players to their B list for the competition if they meet certain criteria.
The speedy 18-year-old forward signed a new contract at United last month after rising through the ranks.
A Sweden youth international, Elanga had been a key figure for Neil Wood's Under-23s prior to his injury and appeared as a substitute in the first team's only pre-season friendly, at Aston Villa, before the 2020/21 campaign kicked off.