The Arsenal star was clattered by San Marino keeper Aldo Simoncini just five minutes into last night's World Cup qualifier.
England boss Hodgson fumed: "Theo is in hospital with a chest injury. It was a fierce attack on him - I don't think for a minute the keeper did it deliberately but it was a rash challenge.
"In normal circumstances he would have been sent off and we would have gone 1-0 up from a penalty earlier on."
Walcott lay motionless on the turf for several moments after the challenge. He appeared to make enough of a recovery to continue but was then quickly replaced by Aaron Lennon.
Hodgson added: "Theo's in hospital and will stay in overnight. He's having scans and tests and we can only hope it isn't too bad.
"It was a penalty that's for sure, it was a very bad challenge."