"I had a bad surprise because it is a hairline crack in his fibula that makes him a few weeks out," manager Arsene Wenger told the club website (www.arsenal.com).
"There is minimal damage apart from the bone damage - there is no damage at all apart from that."
Optimism about the club's most serious championship challenge for many years had been due in part to Wenger having a full squad at his disposal as he attempts to build on two successive FA Cup wins and a third-place finish in the Premier League.
The gifted Wilshere was to be an important part of that continuing progress, but the manager must now do without him for the first few weeks of the new campaign starting this weekend.
The injury, sustained in training last week, will mean another frustrating absence for a player who missed a large part of the last season.
His appearance in the penultimate league game against Sunderland was a first Premier League start for six months and he was only a late substitute in the Cup final win over Aston Villa.
Having made an Arsenal debut aged 16, Wilshere has only once played more than 25 league games in a season and the 23-year-old now finds the label "injury-prone" firmly attached to his name.
The worst period was four years ago, when he missed the whole of the 2011-12 season, including Euro 2012, with a stress fracture of the ankle.
International appearances have been badly affected, despite the backing of successive managers, who have valued a combination of creative passing and tenacious defending ideal in a modern midfield player.
It is that tenacity in the tackle, however, that has often caused problems, resulting in a series of ankle injuries.
This latest blow seems certain to cost him any chance of adding to his 28 caps in England's September internationals against San Marino and Switzerland.
The one consolation for Arsenal, who start their league programme at home to West Ham on Sunday, is that their attacking options remain plentiful.
Alex Oxlade-Chamberlain has returned successfully from injury and Theo Walcott has signed a new long-term contract.
There were signs towards the end of last season that record signing Mesut Ozil was beginning to rediscover his best form, which Santi Cazorla has long been displaying.
Wenger can also call on Welsh international Aaron Ramsey and Chile's Alexis Sanchez, plus back-up from Tomas Rosicky and Mikel Arteta, but Wilshere will still be missed.
(Editing by Amlan Chakraborty)
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