Sunderland's Jermain Defoe earned his first call-up since 2013 after varying fitness issues for Harry Kane, Daniel Sturridge and Wayne Rooney left Southgate short in attack.
Wenger has previously talked up Walcott's potential to operate as a central striker but now accepts his best position is on the right wing.
"I was always convinced, because of the quality of his runs, he was a central striker," he added. "For a while he was as well but he made up his mind during the summer break and he feels more comfortable on the right.
"He is fair. He said if you want me I will play through the middle but if you give me the choice I would prefer to play on the right.
"It is his preferred position and it is true that is where he is most efficient."