Not for the first time, serious injury has struck to leave Arsenal desperately short of options and facing a mad scramble to for signings ahead of the close of the transfer window. This time it is a knee injury to Per Mertesacker that has exposed Arsenal's worrying lack of depth at center-back, threatening to undermine the club's start to he season and its latest attempt to end a now more than 12-year Premier League title drought.

Mertesacker suffered the problem in Arsenal's opening preseason match against Lens last week, with Arsene Wenger revealing that the veteran German will be out for “months.” Arsenal has an immediate crisis on its hands going into Thursday's match against the MLS All-Stars in San Jose. Gabriel Paulista is back in London with tonsillitis, while Laurent Koscielny has yet to return to training after Euro 2016 and looks unlikely to be fit for the start of the season in just over two weeks' time.

With Calum Chambers and new 20-year-old signing Rob Holding the only two fit center-backs and Francis Coquelin looking likely to fill in on Thursday, it is no surprise that Wenger has conceded he is now keen to bring in experienced reinforcement.

“We have to look for an option to get a bit more experience,” he said at his press conference ahead of the MLS All-Star Game. “We will see some of our young centre backs on Thursday as well. With Per being out, we are a bit short on the experience front and we are looking to bring somebody in.”

There is no consensus as yet, however, on where Wenger will turn. Several names have been mentioned, but, as has often been the case with the Gunners, paying the asking price promises to be a significant hurdle.

According to the Daily Mail, Arsenal is one of a number of clubs to have an interest in Kalidou Koulibaly, but is unwilling to meet Napoli's 38 million pounds ($50 million) valuation. Instead Chelsea appears to be in the box seat for the 25-year-old's signature.

The price quoted is certainly a huge one for a player who was highly impressive for Napoli last season, but had a poor first campaign at the Serie A side the year before. A less expensive option could be available.

Shkodran Mustafi, like Mertesacker a member of Germany's 2014 World Cup winning squad, will be made available by Valencia for just 25 million euros ($27.7 million) this summer, reports the Daily Mirror. That is just half the figure of the 24-year-old's buyout clause.

Arsenal is said to have already made an inquiry for a player who started 30 games in La Liga for Valencia last season and also appeared for Germany at Euro 2016. The fact that Mustafi has some experience playing at right-back may further appeal to Wenger.

Arsenal has also been linked with another defender who featured at Euro 2016, in Belgian Jason Denayer. Like Mustafi, Denayer has also operated at right-back. And, after spells on loan at Celtic and Galatasaray, the 21-year-old is now back at parent club Manchester City. However, he admitted on Wednesday that he doesn't know whether he will remain part of new manager Pep Guardiola's squad for the season ahead.

“I'm happy to be back here,” he told Manchester City's official website. “You never know what’s going to happen – it depends on the coach and me. I try to do my best in training and we’re going to see if I’m part of the new manager’s plan or not.”

Denayer would, though, be a strange signing for Arsenal, given Wenger has stated he wants an experienced center-back. Denayer has yet to play a single Premier League game.

Arsenal has made just one major signing in this summer's transfer window: midfielder Granit Xhaka from Borussia Monchengladbach. The lack of a top-class striker has yet to be addressed, with Arsenal so far being rejected by Leicester City's Jamie Vardy and having a bid turned down by Lyon for Alexandre Lacazette.

A new target has now emerged, with Italian daily Gazzetta dello Sport reporting that Arsenal has joined the hunt for Juventus striker Mario Mandzukic. With Juventus having just made Gonzalo Higuain the third most expensive player in history, Mandzukic could certainly find his options more limited this season. And it is believed he could be made available for 20 million euros ($22.2 million) if he pushes for a move, with West Ham and Watford also though to hold an interest.

A target for Arsenal two years ago, the Croatia international would now be a surprise acquisition given that he is 30 years old and arguably wouldn't represent a significant upgrade on current Gunners front man Olivier Giroud.