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 against West Ham and is also likely to be ruled out of 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 suggested that 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, however it swiftly became apparent that 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.
Both ankles have required surgery since he first sustained a stress fracture in his right ankle during pre-season in 2011 and was ruled out for 15 months.
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.
The 20-year-old midfielder is expected to link up with the Baggies after recovering from a serious knee injury which forced him to miss all of last season and he has not played for Arsenal since March 2014.
The 20-year-old has made 18 appearances for the Gunners since joining from Stuttgart in 2011 and scored his only goal in their 2-1 win at Swansea in 2013.
He made his Arsenal debut in a 6-1 League Cup victory over Coventry in September 2012 and made his league bow in a 1-0 defeat to Norwich a month later.
Baggies boss Tony Pulis has been tracking the Germany under-21 international as he looks to add to his midfield options ahead of Monday’s Premier League opener with Manchester City.
Gnabry will become Albion’s fourth summer signing after Rickie Lambert, James McClean and James Chester, with the club also keen on Cardiff and Scotland goalkeeper David Marshall. The Baggies had also been in talks with Bakary Sako, after he lowered his wage demands, before the former Wolves winger joined Crystal Palace on a free transfer on Wednesday.