Walcott had been in decent form for the Gunners, scoring four goals in three games before the international break.
However, the 21-year-old - recalled by Fabio Capello after being omitted from his ill-fated World Cup squad - was taken to hospital last night for precautionary X-rays after turning his ankle in the build-up to Wayne Rooney's opening goal in the 3-1 win over Switzerland in Basle.
Waiting game: Theo Walcott is facing a spell on the sidelines
Initial fears of a break were dispelled by Capello after the match, suggesting the player felt the injury was 'not a big problem'.
However, even if it is just a bad sprain with no torn ligaments, then Walcott - who has a history of niggling injury setbacks - is almost certain to miss Saturday's Barclays Premier League visit of Bolton as well as the Champions League opener against Braga.
Walcott would be unlikely to be risked in the Carling Cup clash at rivals Tottenham, and may have to sit out the trip to Partizan Belgrade - but if all goes well could be fully match fit in time to face title rivals Chelsea at Stamford Bridge on October 3.
Initial fears: Walcott's England team-mates feared the worst
The loss of Walcott is another blow to Gunners manager Arsene Wenger, who has seen Holland striker Robin van Persie sidelined until the middle of next month with an ankle problem.
With Nicklas Bendtner also unavailable because of a groin injury, summer signing Marouane Chamakh looks set to lead the line for the next few weeks.
Midfielder Samir Nasri is recovering well from knee surgery, while defender Sebastien Squillaci made his first appearance for Arsenal following a move from Sevilla in the reserves' win over Blackburn at Underhill and could make his Premier League debut this weekend.
Arsenal £3.5m kid Wellington targets first team after impressive stintLeo's London: Don't blame Avram for West Ham's woes. it's Thomas's faultSquillaci hands Arsenal defensive boost after coming through reserves