Arsenal manager Arsene Wenger confirmed he would look to the transfer market to replace Per Mertesacker despite being unsure how long the defender will be sidelined.
The former Germany international underwent knee surgery after suffering the injury in his side's 1-1 friendly draw against Lens on Friday.
Speaking at a news conference on Tuesday, Wenger said he was uncertain as to how long Mertesacker, 31, would be sidelined.
"We have bad news on Per Mertesacker. He played in Lens on Friday night, he finished the game and had a little pain on his knee," he said.
"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 [exactly] how long he will be out for."
Mertesacker's injury leaves Wenger searching for an experienced defender just weeks before the start of the season.
The Frenchman confirmed he would need to look for a replacement for the 104-time Germany international.
"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."
Wenger's men are preparing for a clash against the MLS All-Stars in San Jose on Thursday.