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 Sunday’s Premier League season opener with West Ham and is also likely out of England’s Euro 2016 qualifying Group E fixtures with San Marino and Switzerland next month.
Wenger said: “I had a bad surprise because it is a hairline crack in his fibula that makes him a few weeks out.”
Reports 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 suffering 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 will be required or not.
Forward Danny Welbeck (knee) will also miss the opening game of the season, along with midfielder Tomas Rosicky (knee).
“We have Welbeck on a progressive recovery and Rosicky on a progressive recovery,” Wenger added. He (Welbeck) is not too far away, but he is not ready for Sunday.” Alexis Sanchez is also unlikely to be included after winning the Copa America with Chile in early July.
“Alexis is back in training but I don’t think he will be involved on Sunday, it is a bit early maybe,” Wenger said.
“He came back in good fitness shape so that is quite positive, but I am not surprised by that because I can’t imagine him lying around for four weeks and doing nothing. That wouldn’t be him.”
Meanwhile West Brom are set to sign Serge Gnabry from Arsenal on a season-long loan. Gnabry has recovered from a serious knee injury which forced him to miss all of last season and he has not played for Arsenal since March 2014.
Gnabry will become Albion’s fourth summer signing after Rickie Lambert, James McClean and James Chester with the club also keen on Cardiff keeper David Marshall.