EXCLUSIVE | By Editor Wayne Veysey

Theo Walcott is set to be sidelined for at least three weeks as he joins Arsenal’s lengthening casualty list, Football Insider sources understand.

The forward pulled up with a calf strain 13 minutes after coming on as a fifth-minute substitute for Alex Oxlade-Chamberlain in the Gunners’ League Cup exit against Sheffield Wednesday on Tuesday.

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Walcott underwent scans on his calf on Wednesday and the club’s medical staff are now awaiting the results.

But Arsenal are not optimistic and sources have told Football Insider that the club expect the forward to be out until after the next international break.

This will rule Walcott out of Saturday’s Premier League trip to Swansea as well as the Champions League visit to Bayern Munich next Wednesday and the north London derby the following Sunday.

It is a shattering blow to the 26-year-old, who had just established himself as Arsenal’s first-choice striker.

The Walcott injury leaves Olivier Giroud as Arsene Wenger’s only senior spearhead for a pivotal run of matches as the Gunners attempt to cement their place at the Premier League summit and gain a foothold in their Champions League group.

Walcott is set to be joined on the sidelines by Oxlade-Chamberlain, who suffered a hamstring injury at Hillsborough.

Speaking after Arsenal had been knocked out of the competition, Wenger admitted the team’s Premier League challenge will be affected by losing the key pair.

He said: “We are short now because we lose Chamberlain today and we lose Walcott. They are two muscular injuries.  We already have a few injuries. How big the damage is, I don’t know as we need to wait 48 hours to have them scanned.

“It is a big blow for us today of course to lose the game, but even more for the other competitions – they are very important to us.

Injury blow: Alex Oxlade-Chamberlain set to join Theo Walcott on sidelines

Injury blow: Alex Oxlade-Chamberlain set to join Theo Walcott on sidelines

“We have a big game on Saturday (against Swansea), a game at Bayern coming up and already we are short, so to lose two offensive players tonight is damaging for us.”

It was a crushing double-blow for Arsenal, who are already without Aaron Ramsey for another three weeks, while Danny Welbeck,  Jack Wilshere and Tomas Rosicky are long-term absentees.