London, United Kingdom: The 122-year-old rivalry between Liverpool and Manchester United enters uncharted territory on Thursday when they meet at Anfield for the first leg of their Europa League last 16 tie.
After 194 encounters in domestic competitions, dating back to 1894, the fierce foes from England's northwest are crossing swords on the continental stage for the first time.
Both Liverpool and United would rather be in the Champions League -- they have been European champions five and three times respectively -- but with neither team assured of a top-four finish in the Premier League, they may need to win the Europa League in order to reclaim a place among the continental elite.
United manager Louis van Gaal has contested suggestions that both clubs are in decline, telling journalists at Wednesday's pre-game press conference: "You live in the past."
"How many titles have Liverpool and how many titles have Manchester United?" he asked. "It was another time. You have to live in the present.
"This happens in football. It's normal. It's not normal that one team dominates for 20 years in a row as champions."
Currently sixth in the Premier League, United have not yet given up hope of a top-four finish, but Van Gaal will be conscious of how damaging elimination at Liverpool's hands could be.
The Dutchman has won all four of his previous encounters with Liverpool, including home and away wins in the league this season, and his side appeared to have turned a corner recently, winning four games in a row.
But Sunday's 1-0 loss at West Bromwich Albion cut them three points adrift of the Champions League berths -- having played a game more than fourth-place Manchester City -- and thrust Van Gaal back into the spotlight.
Liverpool are three points below United with a game in hand after coming from behind to win 2-1 at Crystal Palace, having previously claimed a measure of revenge for their penalty shootout loss to City in the League Cup final with a rousing 3-0 win over Manuel Pellegrini's men.
But whereas United also remain alive in the FA Cup, ahead of a quarter-final clash with West Ham United at Old Trafford on Sunday, the Europa League represents Jurgen Klopp's last chance to end his first season as Liverpool manager with a trophy in his hands.
Mother of all games
Klopp has described the encounter as "the mother of all games".
"These games are easy, because they are so big," said the German, who succeeded the sacked Brendan Rodgers in October.
"You don't have to think about concentration because you are absolutely focused.
"It's big. I don't have to compare games. If you work in (Borussia) Dortmund, as I did, and you play against Schalke, it's not possible to talk about anything else.
"It's similar here with this fixture. It's a really big game. I hope it's not the biggest in my Liverpool career, but it's very important."
United winger Jesse Lingard will miss Thursday's game through suspension after being booked in both legs of the 6-3 aggregate win over Danish side FC Midtjylland in the previous round.
Wayne Rooney's continued absence with a knee injury means that 18-year-old striker Marcus Rashford, who shot to prominence with a debut brace against Midtjylland in last month's 5-1 second-leg win, stands to get his first taste of the Anfield atmosphere.
Antonio Valencia could make his return following a foot injury, but Ashley Young, Luke Shaw, Bastian Schweinsteiger and Phil Jones remain absent, while Adnan Januzaj is cup-tied and Timothy Fosu-Mensah ineligible.
Playmaker Philippe Coutinho and striker Daniel Sturridge are expected to return to Liverpool's starting XI after beginning the victories over both City and Palace on the bench.
Nathaniel Clyne will take over at right-back from Jon Flanagan, who was not included in Liverpool's Europa League squad, but midfielder James Milner is doubtful after being sent home from training due to illness.