Arsene Wenger is unsure when Olivier Giroud will make a comeback from his injury but confirmed the striker will see a specialist about the problem.
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The France international sustained what seemed to be a foot injury in the final minutes of Arsenal's 2-2 draw at Everton on Saturday.
Giroud looked in some discomfort after the ball struck his left foot, and reports on Monday suggested he could face three months on the sidelines.
Wenger confirmed the striker will miss Wednesday's Champions League meeting with Besiktas but did not provide a timescale for his absence.
He said at a news conference: "Olivier will not play against Besiktas and will see a specialist, then we'll make an announcement of how long he'll be out.
"It can be [a long-term injury]. We'll see tomorrow. I don't know [if it's broken]. It's sad for us."
Wenger was then asked whether Giroud's injury would alter his approach to player recruitment in the final week of the transfer window.
He said: "I learn one thing in our job, you say nothing categorically. At the moment we are 24 hours in front of a big game and our focus is on that and not the transfers."
Wenger went on to insist he had no intention of rushing Theo Walcott back into his squad to cover for the probable absence of Giroud, with the England winger closing in on a first- team return as he recovers from a knee injury.
He added: "Theo is doing well but we will respect the timing [of his return]. The timing is end of the month to join in again, after the break I think he will be in full training.
Arsenal skipper Mikel Arteta and full-back Kieran Gibbs will also miss the Champions League playoff game, but Wenger expects both to be available after the upcoming break for international fixtures.
Wenger also said Lukas Podolski will remain at the club despite reports the forward could leave before the close of the transfer window.
Information from the Press Association was used in this report.