• Arsenal defender Per Mertesacker.

Arsenal defender Per Mertesacker. (Photo : Getty Images)

English Premier League powerhouse Arsenal is reportedly set for a massive defensive blow after top defender Per Mertesacker was ruled out "for up to five months" after he injured his knee in last Saturday's friendly match against French club RC Lens.

The Gunners confirmed that the 31-year-old center back is not available for the club's preseason tour in the United States this coming week because of a "knee problem", but did not give out any more details about its severity, according to The Sun.

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However, reports from local newspapers in Spain indicated that Mertesacker could be out until the New Year, the article said, with reporter Kike Marin of El Confidential posting in his social media account that "Mertesacker has a knee injury. He could be 5 months out. Bad luck!"

The 6-foot-6 Hanover, West Germany native also missed Arsenal's final game last term allegedly because of the same problem, but claimed to be "fully fit" after the 1-1 tie with Lens.

Then again, if the rumors that he won't be appearing for the Gunners until January are true, then Arsene Wenger will really have a massive hole to fill-up in his backfield this coming season.

Aside from that, Wenger is also speculated to name Mertesacker as the team's new captain following the retirement of Spanish midfielder Mikel Arteta, who is currently one of Pep Guardiola's assistant coaches in Manchester City, Caught Offside reported.

Without Mertesacker, Arsenal will be left with only one "senior central defender" in Calum Chambers as Brazilian Gabriel Paulista is sidelined by tonsillitis while Frenchman Laurent Koscielny is still on holiday after France reached the final of the Euro 2016.

Newly-acquired defender Rob Holding, who was bought from Football League One side Bolton Wanderers FC last Friday for a transfer fee of £2.5 million, is now set to partner with Chambers during the USA tour.

Club stars Olivier Giroud, Aaron Ramsey, and Mesut Özil are also unavailable for the friendlies after their stints with their respective national squads in the recent Euros, while Alexis Sanchez is also sidelined by injury at the moment.

If Mertesacker's injury will be diagnosed officially as early as possible, then Wenger will have enough time to find a suitable replacement for him while the summer transfer window is still open.