Joe Hugill scored four goals inbetween for Neil Wood's side in a rampant display.
Financial details of the transfer were not disclosed but sources told ESPN that United agreed to pay a base fee of €21 million with €20m in add-ons.
🏁 FT - #MUAcademy U23s: Liverpool 3 #MUFC 6.— Manchester United (@ManUtd) January 30, 2021
A scintillating performance from Neil Wood's side included FOUR goals for Joe Hugill and two for @AmadDiallo_19. Well played, boys! 👏#MUFC pic.twitter.com/LLJ0wo8tbe
"As a club, we have followed Amad for a number of years and having watched him myself, I believe he is one of the most exciting young prospects in the game," United boss Ole Gunnar Solskjaer said.
"It will take time for him to adapt but his speed, vision and fantastic dribbling ability will stand him in good stead to make the transition.
"He's a player with all of the raw attributes that are needed to be an important player for Manchester United in the years to come."