West Ham will be without Jack Wilshere for up to six weeks after the midfielder underwent surgery on his ankle.
The 26-year-old has made a slow start to life at the London Stadium following his summer move from Arsenal, struggling to impress in the Hammers first four games of the season.
Without Wilshere in the squad, the Londoners picked up their first win in the Premier League on Sunday, getting the better of Everton 3-1 on Merseyside.
Boss Manuel Pellegrini will hope his team continue to cope well without the English midfielder after it was revealed he had undergone an operation on Monday to correct an issue with his ankle.
Wilshere, who has missed significant periods with ankle injuries in the past, will be unavailable for some key fixtures with games against Chelsea, Manchester United and Tottenham to come during his period of absence.
West Ham head of medical Richard Collinge says all went well with the operation and hopes Wilshere will be back in action before too long.
"Jack underwent successful surgery in London on Monday morning for a minor ongoing ankle issue," he said in a statement on the club's official website.
"The surgeon was happy with the procedure and all went as planned.
"We envisage he will only need a relatively short period of rest and will then start his rehabilitation. Jack is focused and determined to get back training and playing as soon as possible."
Wilshere joins Andy Carroll and Winston Reid on the sidelines for West Ham, with the veteran duo expected back at some point in October.
Meanwhile, Marko Arnautovic revealed after Sunday's win at Everton that he has a knee injury which is troubling him.
The Hammers will hope the Austrian is available when Chelsea visit the London Stadium on Sunday. The home side are 11/2 for the win, with their London rivals 9/20 and the draw 7/2.
West Ham's first win of the season last weekend saw them move out of the bottom three and they are now 5/1 to be relegated.