Navigating through several relegation scares has ingrained in Wigan a belief that anything is possible. Their composure was clear when, with the game goalless and having been lambasted for playing a short pass that supporters wanted launched, Paul Scharner turned to the crowd, put a finger to his lips and shouted "shush". But that composure disintegrated as a staggeringly inept defensive display gifted victory to Swansea City at the DW Stadium. Belief may soon follow.

Roberto Martínez has manfully tried to separate the pressures of Premier League survival and the first FA Cup final appearance in Wigan's history. The former is the club's lifeblood, the final the reward for a remarkable 35-year journey since being elected to the Football League in 1978. But Wigan will head to Wembley for a date with Manchester City on Saturday with the creeping fear their eight-year residence in the top flight is drawing to a close. It may be a struggle to treat the Cup final as a welcome release following this calamitous home defeat; harder still to collect the wins they will probably need to stay up at Arsenal next Tuesday and at home to Aston Villa on the final day.

The sickening feeling for the majority at the DW was the knowledge it was all so avoidable and that, this morning, Newcastle United should have been under the dotted line. Two seasons ago Wigan collected 15 points from the a possible 30 to preserve their Premier League status on the final day at Stoke City. Last year it was 22 from the concluding 10 matches that did the job. This season Martínez's team have arguably played better and, with 11 points from seven matches prior to Swansea, were on course for another remarkable recovery. But the weaknesses that have undermined that talk of progression returned en masse as Gary Caldwell, Roger Espinoza and company suffered a collective defensive meltdown.

All three Swansea goals arrived as a consequence of defensive errors that cost Martínez's team the win they were on course for when James McCarthy restored their lead in the second half. A poor offside trap from Caldwell allowed Swansea to level once, a dreadful pass from the former Scotland international plus hesitant defending from others gifted the Capital One Cup winners a second, before poor headers from Espinoza and James McArthur enabled Dwight Tiendalli to scramble home an ugly winner in every respect for Wigan. They cannot possibly hope to stay up, or contain the wealth of City's attack at Wembley, with such performances.

All teams end a season with tales of "if only" but few have suffered as many key injuries as Wigan. Ronnie Stam had been on the pitch only a few minutes here when he suffered a suspected broken leg. Central defence has been particularly vulnerable with Antolín Alcaraz plagued by groin trouble and Iván Ramis having his season ended by a cruciate ligament injury in January. It was no coincidence that Wigan's campaign lifted when Alcaraz returned to form a resilient partnership with Scharner, or that the defensive uncertainty that marred the first half to the campaign has returned since he limped off at West Ham with a hamstring tear.

For all the leadership qualities and experience of the Paraguay international's replacement, Caldwell, Martínez must pray Alcaraz recovers in time for Wembley. There is no prospect of a Cup final reward for Maynor Figueroa, who has hardly missed a game in five years as Wigan's left-back but will not enjoy the club's day in the sun having torn a groin muscle in the previous home match against Tottenham. With Figueroa absent and Jean Beausejour hamstrung, it fell to Espinoza to fill the full-back berth. Unconvincingly, it has to be said, despite his fine opening goal.

Espinoza's misplaced back-pass almost gifted Swansea the lead when it sent Wayne Routledge through on the right. Caldwell was almost punished too after Joel Robles gathered his instinctive back-pass inside the Wigan area. The pair were reprieved when Ashley Williams blazed the resulting free-kick over from eight yards with an effort more at home in a Wigan Warriors game.

Wigan's problems in defence were entirely self-inflicted and at odds with the composure shown in the first half from their midfield and attack where, despite being isolated, Arouna Koné's strength and touch kept the Swansea rearguard on edge. The tension in the stands eventually transmitted to the Wigan players and, with survival at stake, they crumbled every time Swansea ventured forth. Caldwell, Espinoza and McArthur were three of the last players to depart, to sporting applause from a crowd that appeared to sense time is up.