Who will come out on top as two of the most successful clubs in England, Liverpool and Manchester United, battle to go on top of the Premier League table?Much has changed in such a short time since Liverpool last met Manchester United in the Premier League. The defending champions registered a 2-0 win then to go 30 points clear of their bitter rivals. Oddly enough, when Manchester United travel to Anfield to take on Liverpool on Sunday, the Merseysiders are the ones playing catch up.Manchester United are on top of the Premier League table and are there for good reason. Liverpool haven't been at their best and their numerous injury concerns have not helped. They are currently three points behind the Red Devils and can go top of the table with a win at home on Sunday.Liverpool 🆚 Man UtdA game like no other 😍 pic.twitter.com/MVTSPj7wn2— Goal (@goal) January 15, 2021Though Ole Gunnar Solskjaer's men have been able to squeeze out three points from dead-rubber scenarios, they will have their work cut out for them at Anfield. It's a trip they will look to make the most out of but it's a fixture that they haven't found much joy in of late.On that note, let's take a look at the five reasons why Manchester United will struggle against Liverpool.#5 Manchester United have not been at their best against the top sidesManchester United v Manchester City - Carabao Cup Semi FinalSave for the 2-1 win against Paris Saint-Germain and the 5-0 thrashing of RB Leipzig in the opening stages of the UEFA Champions, Manchester United have not been able to find much success against top sides neither domestically nor in Europe.They lost to both Paris Saint-Germain and RB Leipzig subsequently. They were held to goalless draws by Chelsea and Manchester City. Two of Manchester United's three league losses came against Tottenham Hotspur and Arsenal. They also lost the Carabao Cup semifinal to Manchester City.So while they have been picking up points against smaller opposition, Manchester United haven't really put their best foot forward against top quality opposition and this is a habit they will have to wriggle out of if they're to remain top of the table after Sunday.#4 Liverpool at Anfield is near impossible to beatLiverpool v West Bromwich Albion - Premier LeagueLiverpool have turned Anfield into an absolute fortress. The Merseysiders haven't lost a single game at Anfield since April 2017. Though they haven't been at their best recently, they would relish the opportunity to throw a spanner in Manchester United's work and put a dent in their title hopes on Sunday.Liverpool have also never lost successive games under Jurgen Klopp and that is an incredible feat. This means that they've never found it hard to get back upon their feet after falling. They faltered against Southampton last week and the players will be charged up to return to winning ways as their old rivals pay them a visit.By the look of things, Manchester United will have to outdo themselves vastly in order to even have a chance of getting something positive from the game.OGS says given Liverpool's status as PL champions and home record it would be 'an upset and a shock' if @ManUtd won at Anfield on Sunday. Says no fans an advantage for his team. "We are the challengers, the hunters."— Simon Stone (@sistoney67) January 15, 2021