Since losing at Chelsea on 23 October Manchester United have become the Premier League’s Invincibles, José Mourinho’s team being the only side to remain unbeaten in more than five months as the season enters its closing phase. Listen to the manager and this has been achieved despite a schedule that chalks up game No49 with West Bromwich Albion’s visit on Saturday. “I don’t think it’s funny some of my players in six weeks play 11 matches,” he had said before the international break.

United face a hardly credible nine matches during April so the manager has a point. Yet the high volume of football is because of United prospering under him. They were in four competitions until winning the League Cup in February and lost in the FA Cup just a fortnight ago. This has all given him time to try players in different competitions to assess who is worthy of the relentless Premier League challenge.

Henrikh Mkhitaryan is one who was substituted on debut at half-time of the September defeat against Manchester City but found a way back into the first team by prospering in the Europa League. The Armenian’s first goal came against Zorya in December, and since the Chelsea defeat a battle-hardened XI largely responsible for the 18-game unbeaten sequence emerged.

Mourinho expertly juggled lineups to ensure the following team operated most regularly in the league: David de Gea; Antonio Valencia, Phil Jones, Marcos Rojo, Matteo Damian; Ander Herrera, Michael Carrick; Henrikh Mkhitaryan/Juan Mata, Paul Pogba, Marcus Rashford; Zlatan Ibrahimovic.

Regular watchers over the 18-match stretch have been accustomed to United conceding very few – 11 in all – while also struggling to score – only 29 times in total. This is why the team, who are fifth, are so far behind Chelsea – 17 points – with eight of their 10 draws coming since defeat at Stamford Bridge. The glaring statistic is the paltry number of goals. In all there have been 42 in 27 league outings. Compare it to Liverpool’s 61, Chelsea’s 59, Tottenham Hotspur’s 55, City’s 54, Arsenal’s 56 and Everton’s 51. Bournemouth, down in 11th, have scored the same amount as Mourinho’s team.

So the key factor in the unbeaten run is a defence in which Rojo-Jones have emerged as the first-choice centre-backs, and Herrera has become a dominant midfielder. Both Eric Bailly and Chris Smalling have sustained injuries and may, in time, dislodge Rojo and Jones. But it is worth recalling Smalling-Bailly were the pair in place for the 4-0 defeat at Stamford Bridge. And Mourinho’s trust in Rojo‑Jones has seen them become two of the six outfield players trusted most during the 18 matches, with 1,262 and 1,225 minutes respectively.

Herrera has played 1,320 minutes (behind only Ibrahimovic’s 1,440, Pogba’s 1,529 and De Gea’s 1,620) and his number of tackles – 43 – is the highest, with Valencia next on 29. The Spaniard has also created three league goals and his 1,202 completed passes is second to Pogba’s 1,345.

Herrera cannot be happy at failing to find the net in 18 games and neither will Pogba be content at only three goals. Mata, who has been allowed only 947 minutes, has chipped in with four.

This again points to the lack of goals being contributed from different areas of the side. Ibrahimovic’s role as totem and chief scorer – he has 11 in 16 appearances – is hardly unknown. Just how much he carries United, though – and why Mourinho is desperate for another elite finisher such as Atlético Madrid’s Antoine Griezmann – is illustrated by the Swede creating the most goals as well. His four assists are more than Wayne Rooney, Pogba, Herrera, Valencia (3), Mata, Anthony Martial (2), and Mhkitaryan (1). Ibrahimovic’s agent, Mino Raiola, is surely pointing to all of this during the negotiations with United over the No9 signing for another year.

Rashford remains the great young hope but can a 19-year-old who has not scored in the league since September be relied upon next season?

Not in doubt is the spirit fostered by the manager. As Jones says: “It [was] good to go into the international break with a win and we are up to fifth. It shows the fight in the squad – we are prepared to fight for each other. You can’t buy that. It is in everyone. It rubs off on us from the manager. Whatever formation we play or the tactics we play, ultimately it is about the fight in every player and we are showing that at the moment. This season we have had times when we’ve had to dig deep; there is a lot of fight in that dressing room and it is positive.”

Mourinho’s team are now about to embark on April’s nine-match endurance test. After West Brom on Saturday at Old Trafford, Everton visit on Tuesday, before United go to Sunderland the following Sunday and travel for the Europa League quarter-final first leg at Anderlecht on 13 April. After that it is Chelsea and Anderlecht (each at home), Burnley and City (both away) before a breathless month closes with Swansea City’s arrival in M16 on 30 April.

As Mata says: “It’s a reality and we have to adapt ourselves, to give the best we have in the decisive spell of the season. I see these games as a challenge to show all the good things we’ve been working on, and I hope we can get good results too.”

United’s current unbeaten run is the best in the division since Sir Alex Ferguson’s side also managed 18 in March 2013. There are seven league fixtures in April. It is a big ask to make it 25 by 1 May.

But that is the challenge for Mourinho’s men as they seek the top-four finish that guarantees a Champions League play‑off place whatever their fortunes in the Europa League, which also offers a route into the competition.