London - Manchester United will be the ones on red alert when Liverpool turn up at Old Trafford on Sunday for a Premier League match neither side can afford to lose.
United were thumped 4-1 by Liverpool at home last March and still went on to win the championship but the stakes are higher than ever now.
The leaders are just two points clear of Chelsea, who have a game in hand and no Champions League to distract them after being dumped out by Inter Milan, and Arsenal as the season enters the final stages.
Liverpool are out of the title picture but Rafa Benitez's side are making an all-out push for fourth and a possible Champions League place and need every point they can get.
They showed their mettle at Anfield on Monday with a resounding 4-1 win against almost-relegated Portsmouth, before turning their attentions to Lille in the Europa League on Thursday.
Liverpool beat United twice last season and will be chasing another double on Sunday after winning 2-0 at Anfield in October.
"It would be very dangerous to think this is a poor Liverpool side coming to Old Trafford because they will be anything but," midfielder Darren Fletcher told the Manchester Evening News.
"It would be a huge mistake to think that Liverpool aren't a threat. They maybe haven't had the season we expected after the one they had last year but they are still a dangerous side.
"They are still the same team they were last year in the main," added the Scot.
"We know what they are capable of because they pushed us all the way and dished out that defeat here and the one at Anfield this season. They are still fresh in our memories.
"We have to learn the lessons from last year's game. The warnings are there. If you are not 100 percent focused and you make mistakes then Liverpool will punish you."
United will hope England striker Wayne Rooney is the one doing most of the punishing after some sensational recent form.
He scored twice against Fulham at Old Trafford last Sunday to take his season's tally to 32 and is on course to surpass the 42 scored by Cristiano Ronaldo in 2007/2008.
While Rooney will be one of the leading lights at the World Cup in South Africa this summer, so too will Liverpool's Spanish striker Fernando Torres who also grabbed a brace against Portsmouth.
Third-placed Arsenal will go top of the league for a day if they win at home to lowly West Ham on Saturday, with Chelsea at Blackburn Rovers on Sunday.
Portsmouth, docked nine points on Wednesday for going into administration last month, host Hull City with Iain Dowie making his debut as manager for the relegation-threatened visitors.