Arsenal boss Mikel Arteta has said the "significant injury" suffered by Gabriel Jesus will bolster the Gunners' efforts in the January transfer window.
Arsenal's marquee pre-season signing was sent home early from the World Cup with a knee problem sustained in Brazil's shock defeat by Cameroon.
Jesus underwent surgery and Arteta, while uncertain of when he will return, said Thursday his injury was severe enough to prompt the current Premier League leaders to look for a new signing next month.
"I never gave a date. I said that we don't know. It's going to take some time for sure," Arteta said of Jesus.
"He's gone through surgery and that tells you the extent of the injury, so we will be in the market and we will assess the biggest opportunities that we have.
"We are going to be active and active means we are looking to strengthen the team."
"This squad still doesn't have the luxury of not maximising every single window. We have to do that because it's really important but then we have to get the right profile. It has to be the right player to take us to the next level."
Following the break for the World Cup, Arsenal return to English top-flight action on December 26 at home to London rivals West Ham with the Gunners five points clear of reigning champions Manchester City.
Jesus has been a central figure in their Premier League progress since his arrival from Manchester City with five goals and six assists, even though he has not scored in his last 10 club games.
"Obviously a huge blow with Gabriel Jesus and his injury," Arteta added on Thursday.
"We'll miss him for a period of time. The season has challenges, that's a challenge we have to overcome, but if you're going to be at the top there's going to be those challenges."
While Jesus faces a prolonged spell on the sidelines, the majority of Arsenal's World Cup players are set to be involved against the Hammers.
William Saliba will return to training on Friday and Arteta will make a late call on the French centre-back but Emile Smith Rowe (groin) and Reiss Nelson (muscle) have both been ruled out.
"Some (World Cup players) came home earlier than expected and they were buzzing to be back and part of it," said Arteta.
"I feel a really good energy around the place. They have got straight away into the rhythm and what we want."