It is an old truism that you can't win a league title with an unreliable goalkeeper and David De Gea produced further evidence of that brutal reality as Manchester United's Premier League ambitions suffered a devastating blow with a 2-1 defeat against bottom club Sheffield United.

The mistakes of one man are never solely to blame for a defeat. There are always other factors involved when a team fails to measure up, but while United lacked creativity, a cutting edge and defensive solidity against a team that had scored just 10 goals in 19 league games prior to their trip to Old Trafford, the fallibility of their goalkeeper was why they finally wound up in a hole they couldn't climb out of.

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With Manchester City grabbing top spot with a 5-0 win against West Brom at The Hawthorns on Tuesday, this game should have been the perfect opportunity for United to reclaim pole position by winning against, by some distance, the worst team in the division. It is not a stretch to hand the Blades that unenviable billing considering they had won just one league game prior to this one and were on course to fall short of Derby County's record low of 11 points from a Premier League season, set in 2007-08.

But despite their nightmare season -- they were even three players short of a full substitutes' bench in this game -- Sheffield United were able to claim their first win at Old Trafford since Dec. 1973, thanks to the home side's inability to overcome the mistakes made by their goalkeeper.

"We had all the possession, but when you concede two bad goals, it's always going to be difficult," United manager Ole Gunnar Solskjaer said.

De Gea was once billed as the world's best keeper, but the Spain No.1 has been unable to reproduce his best form since enduring a mistake-ridden World Cup in 2018. The 30-year-old has had to answer questions about his performances ever since.

Solskjaer's decision to keep Dean Henderson at Old Trafford and deny the 23-year-old keeper another loan spell after two successful seasons at Sheffield United was made to put pressure on De Gea and make him raise his game this season. At times, he has done that, but his shortcomings remain and they were exposed on a bad night for a team harbouring genuine title aspirations for the first time since 2013.

The first goal, on 23 minutes, was down to De Gea. Replays showed that he was pushed off balance by a shove in the back by Sheffield United forward Billy Sharp before Kean Bryan headed in, but De Gea's failure to make a more decisive attempt to deal with a corner left him vulnerable to what followed. A displeased Solskjaer after insisted that 'the goal was a foul -- Sharp ran into De Gea."

De Gea's lack of confidence when dealing with set-pieces in a crowded penalty area is a long-standing problem -- it has cost United more than once this season -- and the visitors made the most of this weakness.

The goalkeeper let United down again in the second half, after Harry Maguire's headed equaliser on 64 minutes should have laid the platform for a home win. On this occasion, a weak and unconvincing attempted clearance fell straight to an opponent inside the penalty area before it was laid off to substitute Oliver Burke, whose deflected strike off Axel Tuanzebe put the Blades 2-1 ahead and ultimately secured their shock victory.

"The second goal we conceded is so poor, so sloppy," Solskjaer said. "Easy. We stopped getting out to the ball, three or four bad decisions, which is out of character."

In between De Gea's two mistakes, United toiled away and struggled to test a team that started the day with a -22 goal difference. Solsjkaer's selection played a major role in that. The United manager chose to leave Edinson Cavani on the bench and start with Anthony Martial as centre-forward, despite the Frenchman, who operates best on the left, repeatedly showing himself to be ill-equipped to play that role.

United were also unable to find a way through a five-man defence that has conceded 33 goals in 20 games. When teams sit deep against United, they consistently cause problems for Solskjaer's team because it is a side that is best when counter-attacking. Quite simply, with Plan A unable to work for United, they didn't have a convincing Plan B and the bottom team was able to keep them at bay.

But despite the combination of issues that led to United's defeat, it was De Gea's mistakes that gave them a mountain to climb and showed why it is so important for a title challenger to have a reliable goalkeeper. It is the fine details that matter and, in this game, those two errors of judgement proved decisive.

Go back through the history of Premier League champions and you won't find many, or any, goalkeepers who let their team down on so many occasions.

De Gea used to fall in that bracket for United, but not anymore.