David Sullivan says it is difficult to understand why West Ham United have struggled to replicate the form they showed during the 2015/16 Premier League campaign.

The London Stadium club secured Europa League qualification with a seventh place finish last season, but have spent much of this campaign battling at the wrong end of the table.

Having retained the core of the squad which performed so well in the club’s last season at Upton Park, co-chairman Sullivan is at a loss as to why West Ham are struggling.

“We haven’t lost any key or influential players since last season, which was one to remember, so we are all currently struggling to understand why the team is under-performing,” he wrote in his column on the club website.

“We know we need to improve quickly, and upcoming fixtures against Crystal Palace and Middlesbrough now become cup finals.

“I know it can be difficult at times like this, but I urge every supporter to get behind the team and manager – just as you have done all season.”

West Ham’s resurgent form, which saw them go four games without defeat over the festive period, has stalled with the turn of the New Year.

Defeat against Manchester City in the FA Cup extended the Hammers’ losing run to three games in all competitions.

Sullivan though insists United can recapture the form of last month and believes Saturday’s clash with Palace provides the perfect chance to start improving.

“It’s another London derby against Crystal Palace, which brings back memories of our 1-0 win in the reverse fixture at Selhurst Park, where we began to turn our season around earlier in the campaign,” he said.

“We have seen performances improve recently, even in defeat at Leicester City and against Manchester United, so I truly believe we can continue to progress.

“As we showed over Christmas, a string of good results can move you up the Premier League table very quickly, and I am confident we will be heading in that direction over the coming weeks.”