UPDATE: Friday, March 22 at 3:40 p.m. ET by Tim Daniels
Arsenal confirmed on the official club site that Walcott was removed from the England squad due to injury and is heading back to get assessed by the medical staff. The Gunners' preliminary word is that it's a minor hip or groin injury. The Daily Mirror called it a thigh ailment.
More will be known after he's tested by the club's doctors.
---End of updated---
The report states Walcott's injury is not believed to be serious, but it was bad enough to send him back to England as the squad gets prepared for road matches on Friday and Tuesday. The Arsenal star was expected to play a key role for the Three Lions.
Instead, he will return to Emirates with hopes of getting fit in time for the Gunners' clash with Reading next weekend. While his status for that match remains uncertain, the following week against West Brom is a safer target, according to the report.
Walcott is enjoying a breakout season for Arsenal with 18 goals and 15 assists across all competitions. England manager Roy Hodgson was hoping he would carry that form over to international duty as the Three Lions look to get back on track.
They currently sit in second place in Group H of UEFA qualifying after two wins and two draws in their first four matches. That leaves them two points behind Montenegro for the automatic qualifying position for next year's World Cup.
England should be able to handle San Marino without Walcott. The overmatched nation has allowed 16 goals and scored none in its four qualifiers to this point. Anything less than three points in Friday's clash would be a disaster for Hodgson's side.
It would have been nice to have Walcott available for the key clash with Montenegro, though. The pace he brings to the outside is irreplaceable for the Three Lions and will force Hodgson to make some tactical choices before Tuesday.
England doesn't want to fall any further behind Montenegro, so it needs to find away to put together a strong pair of showings without him.