Walcott was sent back to Arsenal after he fell ill earlier this week.
"Theo Walcott was the catalyst for us to seek out a contingency plan because it's some sort of virus," explained Hodgson.
"He had trained fully on Sunday and was as fit as a fiddle but was then violently sick on Monday morning and had to be sent home.
"The doctor is quite confident he will be OK, though. I can only hope nothing is going round the squad. That was why we were quite keen to get Theo home as soon as possible."