Arsenal midfielder Jack Wilshere is facing up to five months out after undergoing ankle surgery.
The 19-year-old sustained the injury in the Gunners' pre-season friendly against New York Red Bulls on 31 July.
It was only expected to rule him out for a short while, but has since been diagnosed as a stress fracture.
A statement on the club's official website read: "It is envisaged that Jack's rehabilitation will take a period of four to five months."
The England international had earlier written on Twitter: "Had my operation and all went well! I am feeling good, will spend the night in hospital to monitor me but apart from that I am happy!"
And he added: "I've had my operation this morning and all went well. My ankle is little bit sore, but I'm already looking forward to working hard to get fit and playing again.
"I would also just like to say thanks for the many messages of support I have received from you - the fantastic Arsenal supporters. See you all soon at the Emirates."
Wilshere's absence adds to a disappointing start to the season at Emirates Stadium, which has seen them lose three of their six Premier League matches, although they climbed up the table with a .
Wilshere had been wearing a protective boot to guard the affected right ankle, which manager Arsene Wenger claims was first damaged when the midfielder was on England duty against Switzerland in June, in an attempt to avoid surgery.
Wenger won his running battle with England Under-21 counterpart Stuart Pearce over Wilshere's selection for the summer's European Championships in Denmark, but the rest he was able to enjoy in the close season was not enough to cure the problem.
Arsenal's statement on the surgery added: "Jack Wilshere has this morning undergone ankle surgery at a central London hospital.
"Under the care of two world-renowned surgeons, Jack has had the stress fracture of his right ankle repaired.
"The initial indications are that the surgery has been successful and Jack will now undergo a prolonged period of rehabilitation."