Manchester United have let themselves become reliant on David de Gea's goalkeeping masterclasses

Manchester United can count themselves lucky that the member of Jose Mourinho’s squad that most is demanded from is also their most reliable player.

‘Dave saved’ in Seville, as he did at Anfield and the Emirates earlier in the season, as he has on countless other occasions since gradually developing into the world’s finest goalkeeper.

Wednesday night saw yet more clips added to David de Gea’s reel of career highlights. Two breath-taking stops at the end of the first half were the new entries, both straight into the all-time top 10.

If the first to tip Steven N’Zonzi’s header over the crossbar was not impressive enough, the next a mere minute later was one for the ages. To deny Luis Muriel from six-and-a-half yards out, De Gea reacted in just 0.28 seconds, palming the ball up and away with his right hand.

His team-mates, manager and supporters alike hailed another preternatural performance after the final whistle, like they have so many times before, but this provoked an obvious question: how long can De Gea, and by extension United’s defence, keep this up?

The draw in Seville was, after all, yet another example of the Spaniard single-handedly keeping his team competitive and that has happened too often for a team of United’s supposed talent this season.

De Gea has particularly come to the rescue against other top clubs, when he has not merely had to play well but be practically super-human in order for United to avoid defeat.

Mourinho’s side took a goalless draw from Anfield in October, for example, but only thanks to their goalkeeper’s heroics.

Joel Matip came closest to putting Liverpool ahead that day, and a better finisher would probably have scored, but that took nothing away from De Gea’s denial of the defender as he superbly blocked the goalbound effort with his left foot.

That stop might have been the Premier League’s save of the season were it not for De Gea’s display a month later against Arsenal, one of the great goalkeeping display of recent years. The performance, littered with barely credible saves, was one of a goalkeeper maximising his potential, in the form of his life.

Now this draw with Sevilla and that spectacular stop to deny Muriel can join the list.

But compare the aforementioned results with the defeats away to Chelsea and Tottenham Hotspur – 1-0 and 2-0 respectively.

De Gea did not play badly in either game, far from it, but he was not in the type of spectacular form that has saved United on other occasions. Even he could not prevent Alvaro Morata’s inch-perfect header, Christian Eriksen’s sucker punch inside 11 seconds and Phil Jones being Phil Jones. Against Manchester City at Old Trafford, he was failed by the individual errors of those in front of him too.

His only relatively easy afternoon against the big boys this season came in October against a Tottenham side without Harry Kane. On every other occasion, De Gea has been called upon frequently by the opposition’s attack and unless at his best, United have lost. That should cause some concern ahead of Chelsea’s visit to Old Trafford this weekend.

United still boast the Premier League’s meanest defence, with just 19 goals conceded and De Gea’s total of 15 clean sheets is the highest of any current top-flight ‘keeper too, but the underlying numbers have suggested for some time that if anything, Mourinho’s lukewarm football has been running a little hot.

The metric ‘expected goals against’ (xGA) measures the quality of chances that teams concede and according to one model, United only have the top flight’s fifth-tightest defence. Their total of 31.74 goals is greater than those of City, Tottenham, Chelsea and Liverpool.

The large discrepancy between this number and United’s actual 19 league goals conceded cannot be solely put down to the excellence of De Gea – there is such a thing as shots off target and poor finishing, for a start – but it does give a indication of just how much it helps to have the best goalkeeper in the world between the sticks.

It also indicates that those in front of him do not offer enough protection. Even if Eric Bailly makes a welcome return this weekend, he is unlikely to immediately answer the questions that have recently been levelled at United’s backline. Chelsea, meanwhile, look like a team ready to put a difficult spell behind them. Mourinho may require another De Gea spectacular on Sunday.