With United already through to the last 16 of the competition and unlikely to usurp Juventus at the top of the group even with a win in Spain, Jose Mourinho could name an experimental team for the match at the Mestalla.
Neither Greenwood nor Garner have played a senior match for United before, but the club confirmed on their official website that both of them have been training with the first team, and both have flown out for the match.
Striker Greenwood was the top goalscorer for the Under-18s side last season and recently scored on his Under-23 debut, while Garner is a central midfielder who has also been used as a centre back.
It remains to be seen what strength of side Mourinho will put out on Wednesday. Eric Bailly, Luke Shaw and Anthony Martial missed the Premier League win over Fulham on Saturday, while Chris Smalling was taken off in the second half with an apparent injury.
Mourinho is unlikely to take any risks with a crucial Premier League game to come against Liverpool next weekend, although he has confirmed that Paul Pogba, Scott McTominay and Andreas Perreira will feature at the Mestalla.
Marouane Fellaini’s late winner against Young Boys on matchday 5 booked United’s place in the knockout stages and eliminated Valencia, who will drop into the Europa League. United will win the group if they beat Valencia and Juventus fail to win against Young Boys.