West Ham have confirmed Jack Wilshere has undergone surgery on his ankle.
Sky sources understand the midfielder will miss up to six weeks with the injury.
The 26-year-old, who joined the Hammers after leaving Arsenal at the end of his contract in the summer, was ruled out with the problem prior to Sunday's 3-1 win at Everton.
Wilshere will now miss key fixtures against Chelsea, Manchester United and Tottenham as West Ham attempt to climb the Premier League table following a slow start to the new season.
He has played four times for the Hammers this season, with his last appearance coming in the 1-0 defeat to Wolves at the London Stadium on September 1.
West Ham head of medical Richard Collinge has confirmed the operation went as planned.
"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."
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