Ray Parlour says Theo Walcott's season-ending injury is a "massive blow" for Arsenal and England.
Ex-Arsenal star Parlour believes Gunners boss Arsene Wenger may struggle to find a replacement for the England winger, whose cruciate knee ligament injury - suffered in the 2-0 win over Spurs on Saturday - will rule him out of the World Cup.
Parlour believes the return of Alex Oxlade-Chamberlain to fitness will offset the loss of Walcott up to a point, but he doubts Wenger will be able to find a like-for-like replacement in the January transfer window.
"When he was stretchered off at the weekend he looked to be in pretty good spirits and no-one expected an injury like he's now got," Parlour told Sky Sports News.
"The scan results are terrible news - he'll be absolutely distraught this morning because he was just coming back into form and everything was going really well and he could have looked forward to the World Cup in Brazil.
"Theo was always an outlet for Arsenal - when you're up against it as a team you can stick it behind the full-back and he's onto it like a shot with his pace.
"His finishing is also so much better than it was a couple of years ago - his all-round game has got better and he's at a good age - 24 - and he's getting more experienced.
"You look at the areas where Arsenal might struggle - it's up front with (Olivier) Giroud and (Nicklas) Bendtner also out at the moment, (Lukas) Podolski is their only fit striker - it's a big, big problem for Arsenal now.
"Alex Oxlade Chamberlain is an obvious replacement - he also plays well centrally in a midfield three, he's a real talent - but Theo was just starting to click into form.
"I'm sure Arsene Wenger will take a look at that position because Oxlade-Chamberlain has been out for a long period of time.
"Cruciate injuries are pretty common - I played with Robert Pires who had it in our unbeaten season. You just have to get through it - coming back from an injury like that is a lonely road - you're on your own, with a fitness instructor every day. You see the lads going off to train and you can't be involved so it's going to be a long period for Theo.
"The big question now is who's available (to sign) - I don't think managers really like doing business in the January window - they prefer to get players in during pre-season and bed them into the team.
"The likes of Dimitar Berbatov have been mentioned - he's in the last year of his contract at Fulham and Arsenal could probably get him on the cheap - that might be an option.
"People take about Diego Costa - he's having a great season at Atletico Madrid. I think he's got a 40m euro clause in his contract. But then Atletico are going really well in La Liga and in the Champions League so whether they'd want to let players go, we'll have to wait and see."
"There have been plenty of names bandied around but Arsene Wenger's got to get the right man - and he's got to get the competition right as well because Giroud should be back fairly soon - he's done really well but he could do with a bit of competition to keep him on his toes.
"It's a massive blow for Arsenal, for England - and for Theo."
The Gunners' worst fears were confirmed just a few hours after the Football Association decided the player would face no disciplinary action for his hand gesture.
"The club regrettably announces that Theo Walcott has sustained a ruptured anterior cruciate ligament of his left knee," Arsenal said in a statement.
"Theo will have surgery in London in the near future and is expected to be out for at least six months, therefore missing the remainder of the season and the World Cup in Brazil in the summer.
"Everyone at Arsenal wishes Theo a speedy recovery and we look forward to seeing him back on a football pitch as soon as possible."
The full diagnosis of Walcott's knee problem is now likely to have a knock-on impact to Wenger's plans on how best to bolster his squad during the January window in an attempt to last the distance in the battle for honours.
The attacking options of the Barclay Premier League leaders were already depleted, given Bendtner is expected to miss all of January with the ankle problem he picked up while scoring a late goal in the 2-0 win over Cardiff on New Year's Day, while France Under-21 forward Yaya Sanogo is also way short of match sharpness.