Arsenal could be facing a defensive crisis before the Premier League season even starts, ramping up the urgency for manager Arsene Wenger to sign an experienced centre-back in the transfer window.
Per Mertesacker sustained a knee injury that has ruled him out of this week's preseason trip to the United States, with reports claiming that the German could be out for up to five months.
But while the club has yet to reveal the extent of the injury, fellow centre-back Gabriel Paulista also stayed home this week with tonsillitis and Laurent Koscielny will return late from his post-Euro 2016 holiday, leaving young England duo Calum Chambers and Rob Holding (just arrived from Bolton) as the only two fit centre-backs.
Wenger made it clear over the weekend that he's still looking to sign a defender this window, so here is a look at five targets he could go for.
Kostas Manolas - Roma
The Greece international has been the subject of constant transfer rumours this summer, with Chelsea supposedly also interested in his services. Manolas has the size and strength to handle Premier League attackers but is also deceptively quick for a 6-foot-2 centre-back. Indeed, he has been hailed as the leader of Roma's back four and would be an instant upgrade for Arsenal alongside Koscielny. However, Roma coach Luciano Spalletti has insisted that the 25-year-old will stay at the club, and any deal would involve a hefty transfer fee.
Shkodran Mustafi - Valencia
After an impressive season at Valencia, the Germany international has reportedly emerged as one of Wenger's main targets. While he's only 24, Mustafi is good on the ball and has plenty of experience as he helped Germany win the 2014 World Cup and scored at Euro 2016. He also has experience of playing in England as he spent three years at the Everton academy, although did not make a Premier League appearance, and is well suited to Arsenal.
Kalidou Koulibaly - Napoli
The towering Senegal defender is another player likely to spark a bidding war this summer after his agent said he wants to leave Napoli. Koulibaly, 25, has been primarily used as a central defender but can also be deployed on the right and offers the type of versatility that Wenger values highly. And at 6-foot-4, he would strengthen the Gunners in the air, one of their biggest weaknesses last season. But his transfer fee could be a big sticking point as Napoli are reportedly asking for £37 million -- more than twice as much as Wenger has ever paid for a defender.
Jeison Murillo - Inter Milan
The 24-year-old was named Best Young Player at last year's Copa America after impressing with Colombia and had a solid season at Inter Milan. The Italian side could be tempted to cash in for a quick profit on their asset, and Italian newspaper Gazzetta dello Sport claims the Gunners are interested.
Mathias Ginter - Borussia Dortmund
The 22-year-old will be key to helping Borussia Dortmund overcome the departure of Mats Hummels next season -- if the German club can hang on him. Coach Thomas Tuchel has acknowledged that English clubs are interested, but also insisted the club have no intention of selling. However, if Arsenal could strike a deal, Ginter would be a natural long-term replacement for Mertesacker and could spend a couple of years learning from his experienced countryman.