In April, West Ham United were running Manchester's two top sides close in the hunt to qualify for Champions League Football.

Slaven Bilic's side had to settle for the preliminary stages of the Europa League in the end but, with the anticipation of moving into a modern, 60,000-seater stadium and a bright, young team and manager entertaining and impressing, life in the East End of London was looking rosy.

But after a week in which they were dumped out of the League Cup at Old Trafford and then humiliated in front of their own fans 5-1 against an Alexis Sanchez-inspired Arsenal to leave them languishing just a point outside of the relegation zone, the Hammers' optimism has been replaced with huge disappointment - and this has been the catalyst for ugly scenes in the stands of the London Stadium.

So why have West Ham suffered such a dramatic downturn in form in just a few months?

Contrasting transfer windows

During the summer prior to last season, West Ham celebrated the arrival of Dimitri Payet for a staggeringly low £10.7 million. Manuel Lanzini was plucked from the obscurity of the UAE Arabian Gulf League and Michail Antonio was bought from Nottingham Forest. This trio was the key to a successful season and scored 39 goals since arriving in east London.

If you compare that to the pre-season just gone, you can see why things have gone wrong.

Andre Ayew was signed for a club-record fee of £20.49m. He lasted 35 minutes of his debut before getting injured and has failed to regain a starting spot since his return in October.

One area the Irons clearly needed to strengthen was at full back, with Aaron Cresswell and the inexperienced Sam Byram the only options. But Bilic dallied and had to settle for Arthur Masuaku from Olympiacos for £6m and free agent Alvaro Arbeloa. Both have been at fault for several goals conceded.

The club signed two right wingers and both have flopped.

Sofiane Feghouli looked a decent acquisition from Valencia on a free and impressed in the early Europa League fixtures. But he has since struggled with the intensity of the Premier League, as has Gokhan Tore who was also booed by West Ham supporters for his lack of effort.

Edimilson Fernandes has shown glimpses of talent but is being played in positions that seem alien to him, at wing back or in defensive midfield ahead of Havard Nordtveit, who has fallen out of favour since giving away a crucial penalty in the agonising defeat at Tottenham.

However, the most costly bit of poor business has to be in the striking department. The first mistake was deeming Diafra Sakho surplus to requirements. Yes, the Senegalese is injury-prone, but he has been West Ham's most natural goalscorer for the past few years. So with Sakho sulking, the need for competition to also injury-prone Andy Carroll was clear for all to see.

But Carlos Bacca and Michy Batshuayi shunned the Hammers and, again, West Ham were forced to panic-buy. In came Argentine Jonathan Calleri on loan and he has looked OK until anywhere near the goalposts. Candidate for worst penalty-taker of all time Simone Zaza also joined. But he has struggled to cope with the ever-changing clause in his contract detailing the number of games before his switch from Juventus is made permanent for another £20m - and has failed to score in 11 appearances.

Ashley Fletcher is the only positive addition to the squad but he is only 21 and can't be expected to fire West Ham to safety in his breakthrough season.

Tactical switch

Bilic has changed the formation from 4-2-3-1 to 3-4-3. To an extent this has worked, with Cresswell and Antonio becoming more involved and full-back issues resolved. But West Ham are still being caught out defensively and by playing star man Cheikhou Kouyate at centre back, his box-to-box running is missed and makes the side less of a force on the counter-attack.

West Ham are paying for poor signings and it is imperative they have a successful January window. Otherwise, they will be considered real relegation candidates.