The numbers behind Barca, United, Bayern and Madrid's struggles

Barcelona, Real Madrid, Manchester United and Bayern Munich are all struggling for form in 2018-19, and Opta's stats tell us why.

Manchester United manager Jose Mourinho   -  Getty Images

From the crisis engulfing Jose Mourinho at Manchester United to Barcelona and Real Madrid trailing Sevilla in LaLiga, the 2018-19 season has seen some of Europe's top clubs struggle.

While the board at Old Trafford are reportedly contemplating Mourinho's future, pressure is also building at the Santiago Bernabeu where Julen Lopetegui is without a win in his last four matches in all competitions.

Reigning LaLiga champion Barcelona is in a similarly indifferent spell of form, while Bayern Munich is sixth in the Bundesliga following a humiliating 3-0 defeat at home to Borussia Monchengladbach in its last outing.

Opta's stats provide a telling insight into the key areas in which these four giants of European football are failing in this campaign when compared to their performances last season.

READ: Rooney warns Man United players to 'stand up' amid Mourinho criticism

Errors blighting Barca progress

Barcelona's 4-2 Champions League win over Tottenham will have eased the pressure on Ernesto Valverde, who has watched his team stutter in the league in recent weeks.

With three draws and a defeat from their last four league games, Barcelona's performances have been undermined by an increase in defensive errors.

Valverde's men made just one error leading to a goal throughout last season, but this campaign, they have already committed three.

At the other end, Barca's shot-conversion rate has dropped from 17.1 last season to 13.8 this season, and it is averaging 2.4 goals per game compared to 2.6 in 2017-18.

ALSO READ: Lopetegui's Madrid must find balance without Ronaldo – Emery

Misfiring Bayern beginning to flounder

After starting the season in fine form, Bayern is on a downward spiral, taking one point from its last three Bundesliga games.

Boss Niko Kovac will be asking questions of his forwards after Bayern failed to find the net in consecutive league defeats to Hertha Berlin and Borussia Monchengladbach, and it has gone from averaging 2.7 goals per game last season to 1.7 this season.

Bayern turned 15.3 per cent of its shots into goals in 2017-18, whereas this season, its conversion rate is just 9.8, while its expected goals per game (xG) has dropped from 2.3 to 1.8.

ALSO READ: Mourinho criticism becoming 'personal' – Neville backs United boss

Defensive weakness hampering Mourinho

Mourinho's Manchester United future looked bleak at half-time of his side's Premier League clash with Newcastle United on Saturday, but second-half goals from Juan Mata, Anthony Martial and Alexis Sanchez delivered a 3-2 victory.

The win moved Mourinho's men up to eighth in the Premier League after eight games, following a hugely disappointing start to the season that has seen it perform worse than it did in 2017-18 in a number of key metrics.

United is averaging 1.6 goals per league game this season, compared to 1.8 last season, and its shot conversion rate has slipped from 13.3 in 2017-18 to just 11.6 this term.

But, a more porous defence has been perhaps a bigger issue for Mourinho's side, which has conceded an average of 1.8 goals per game this season, compared to 0.7 last campaign, and faced an average of 4.8 shots per match, compared to 3.9 the previous season.

ALSO READ: Bayern president Hoeness vows to defend Kovac 'until the end'

Madrid goals hard to come by in post-Ronaldo era

Lopetegui must have thought he had inherited an easy life when his Real Madrid side clocked up its fourth win in five LaLiga games with a 1-0 victory over Espanyol on September 22.

However, since then, Los Blancos has slipped to away defeats at Sevilla and Deportivo Alaves, either side of a draw at home to Atletico Madrid, and the stats tell an unhappy story for Lopetegui.

Under Zinedine Zidane - and with Cristiano Ronaldo in the team last season - Madrid averaged 7.5 shots on target per game, but that number has dropped to 5.6 this season, with its conversion rate also dropping from 13.5 to 9.2.

The result has been a decline in the team's average goals per game from 2.5 to 1.5, and when margins are as narrow as they were in Madrid's defeat to Alaves, that change can be crucial.