Arsenal boss Arsene Wenger has confirmed Per Mertesacker's knee injury means he will be looking for a new central defender.
The 31-year-old suffered the problem in Friday's pre-season draw with Lens and did not travel with the squad for two friendlies in the United States.
Wenger confirmed on Tuesday evening that Mertesacker will be out of action for some time after having a knee operation.
"We have bad news on Per Mertesacker," Wenger said on the official Arsenal website ahead of a clash with the MLS All-Stars on Thursday.
"He played in Lens on Friday night, he finished the game and had a little pain on his knee. We found out the next day that it was much more serious than expected.
"He had surgery yesterday in Germany. It all went well. I know you will ask me for how long he will be out. I don't know that, it's months, but I don't know how long he will be out for."
The injury comes as a blow to Wenger, who also left fellow centre-back Gabriel Paulista in England and has given Laurent Koscielny an extended break following his exertions with France at Euro 2016.
That means 21-year-old Calum Chambers is his most experienced defender on the US tour, with new signing Rob Holding also expected to feature in both pre-season matches in America.
But with the new season looming, Wenger admits the absence of Mertesacker means he will be forced to bring in a replacement in the current transfer window.
"We have to look for an option to get a bit more experience," Wenger said.
"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."
But Wenger was in no mood to offer up any suggestions of who he may be tempted to bring in after responding to a statement from Lyon which claimed they had rejected a bid from the Gunners for striker Alexandre Lacazette.
"Lyon is responsible for their own statements," he said.
"On the transfer front, I think the best option is always to keep as secret as possible. If we sign somebody, we will announce it. But I cannot tell you much more on that."