Rooney was ruled out of Manchester United's 1-0 defeat to Liverpool at Anfield after suffering a gash in a training ground accident.
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"I have seen a picture and it is a very, very big gash," the Arsenal forward told reporters. "It's not nice to see. It's like something out of a horror film."
Despite the absence of the Old Trafford striker, and Daniel Sturridge being afforded extra time to recover from a thigh problem, Walcott is confident England have plenty of attacking options for a pair of crucial qualifiers.
He continued: "It is sad to see [Rooney miss out] but it is an opportunity for someone to come in and take their chance.
"It is great to see Danny [Sturridge] still here at the moment, that's a positive. Danny Welbeck has started the season well for his club and Rickie Lambert has had a great start to his England career, so the manager has some great options."
England host Moldova at Wembley on September 6, before travelling to Ukraine to face a side just one point behind them in Group H four days later.