The 23-year-old England midfielder's career has been punctuated by injury and now he has suffered another - a hairline crack of his left fibula.
Wilshere will miss Saturday's Barclays Premier League season opener with West Ham and also faces missing England's Euro 2016 qualifying Group E fixtures with San Marino and Switzerland next month.
Wenger said on arsenal.com: "I had a bad surprise because it is a hairline crack in his fibula that makes him a few weeks out."
Reports had suggested Wilshere could be out for around two months, but Arsenal have not put an exact timescale on the injury yet
Wilshere missed last Sunday's Community Shield defeat of Chelsea, the Premier League champions, after sustaining the injury in training the previous day.
Wenger initially said it would be a "matter of days" before Wilshere returned, but it swiftly became apparent he would miss the Hammers clash - and now the prognosis is far worse.
It is the latest worrying blow for Wilshere, who returned in May following another lengthy spell out with an ankle problem and has had numerous injuries throughout his career.
Wenger added: "There is minimal damage apart from the bone damage - there is no damage at all apart from that.
"It was a collision in training and it was all completely accidental."
Wilshere will continue to be assessed and treated by FA Cup winners Arsenal and there was no mention of whether surgery would be required or not.
Source : PA