Teenager Mastny, who hails from the Czech Republic, is a fourth keeper in the official squad, alongside David De Gea, Dean Henderson and Lee Grant.
Mengi played for the Under-23s on Saturday against Arsenal but was promoted to the senior squad at the start of the season. Williams and Greenwood are long-established members of Ole Gunnar Solskjaer's side and both started last season's Europa League semi-final with Sevilla, of course, in our last European outing.
Solskjaer may provide more information on his actual travelling party when he holds a press conference in Paris around 18:00 BST.
An extended 23-man matchday squad can be named as five substitutions are permitted in the competition.