Michael Carrick's side were without Kobbie Mainoo, Luke Shaw, Ayden Heaven, Lisandro Martinez and Marcus Rashford during Tuesday morning's session at Carrington.
Mainoo, Shaw and Heaven also missed the weekend's 1-1 draw against Leeds United, while Martinez was withdrawn at half-time after picking up a thigh injury.
Rashford, meanwhile, remained on the bench throughout the Leeds encounter, with Carrick previously revealing that the forward had been managing a knock.
"Marcus has been carrying a little bit of something so we're managing him to a point," said Carrick.
Benjamin Sesko, Patrick Dorgu and Manuel Ugarte were also absent as they continue their respective long-term recoveries, while third-choice goalkeeper Tom Heaton did not train.
Leny Yoro is certain to miss Wednesday's fixture after being sent off during United's defeat to Atletico Madrid in the Spanish capital.
The Red Devils will be seeking a response to that setback as they resume their European campaign.
The encounter at Stadio Giuseppe Sinigaglia, situated on the shores of Lake Como, will be the first competitive clash between the two clubs. UEFA approved the Italian venue for the match, where United will meet Cesc Fabregas' side.
Liverpool boss Andoni Iraola has revealed that Villarreal counterpart Inigo Perez was his preferred choice to join his coaching staff before a work permit rejection prevented the Spaniard from moving to Bournemouth in 2023.
Cesc Fabregas believes Como's upcoming meeting with Manchester United represents a special occasion as the Italian side prepares for its first-ever Champions League campaign.
Harry Maguire was left frustrated after Manchester United's ongoing problems defending set-pieces resurfaced during Sunday's 1-1 Premier League draw against Leeds United at Elland Road.
Dominic Calvert-Lewin's clinical second-half finish rescued a point for Leeds United as they fought back to draw 1-1 with Manchester United in a fiercely contested Premier League clash at Elland Road.
