United were paired with Premier League strugglers Hull City after defeating West Ham United 4-1 at Old Trafford, three days on from a 1-1 league draw between the teams at the same venue.
Hull, who were promoted via the Championship play-offs at Wembley last season, saw off Championship leaders Newcastle United on penalties on Tuesday.
"If the teams arrive here, it is because they put a lot on the competition," said United manager Jose Mourinho.
"I am sure that Hull City has an amazing memory from Wembley from last season. I'm sure that they are going to try everything to be there again."
Liverpool, 2-0 victors against second-tier Leeds United on Tuesday, will face Southampton, who pulled off a shock 2-0 win at Arsenal on Wednesday.
English League Cup semi-final draw, made on Wednesday (two-legged ties to be played in January):
Manchester United v Hull City
Southampton v Liverpool
Original source: Man Utd, Liverpool kept apart in League Cup