Arsenal manager Mikel Arteta revealed the club has not set a timescale on Gabriel Jesus' return from injury.
Brazil striker Jesus has been an ever-present in the league for Arteta's side, scoring five goals. He is not expected to be available for the Gunners immediately after the World Cup, though, thanks to an injury sustained on international duty.
Arsenal sit five points clear at the top of the Premier League, with Jesus' former club Manchester City their closest challengers.
There has been talk of January transfer activity, though, with the 25-year-old out of action and team-mate Emile Smith Rowe yet to recover from an injury issue of his own.
"‘We know that he needed some intervention in the knee and we have to take day by day, week by week and see where that takes us," Arteta said. Early reports suggested the striker could be out for between three and four weeks, but the Arsenal have since confirmed he has required surgery on the problem.
"Gabby will now begin his rehabilitation programme," a club statement confirming the surgery read. "Everyone at the club is supporting Gabby and will be working hard to get him back on the pitch as soon as possible."
Arsenal are due to return to Premier League action on December 26, at home to West Ham. They will then travel to Brighton on December 31, with the transfer window opening the following day and allowing Arteta to look for reinforcements.
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"What it affects is who we are as a team because he gives us so much," Arteta added. "What we can do after that when we know the timescale better and we have when we can have him back we will look at the options and try to make the right decisions.
"We look at the squad and see what the options we have our with the players we have available. There are things we can try, we have the need to do so.’"
Arsenal started with Eddie Nketiah up front as they took on Lyon in a mid-season friendly. The academy graduate was on the scoresheet against the Lyon side, as were Gabriel Magalhaes and Fabio Vieira, as Arteta's team ran out 3-0 winners.
Earlier, Jesus' club colleague Takehiro Tomiyasu sent a message of support for the Brazilian. Defender Tomiyasu played three times for Japan during the World Cup, including a start in the last 16 as Hajime Moriyasu's side suffered a penalty shoot-out defeat to Croatia.
"It's a shame, because obviously to play in a World Cup is a big dream for us, for every football player,” he said. “So I'm sorry for him, but hopefully he will be back soon."
