Asked to describe the injury during a press conference at England's St George's Park training base on Tuesday, Walcott pointed to a bottle of water beside him and said: "I've seen a picture of it and it's about that big.
"It is a very big gash. It is not a nice sight, to be honest. It is not going to help his looks, I wouldn't think. It's not very nice to see. It's like something out of a horror film."
Liverpool striker Daniel Sturridge had been in line to deputise for Rooney at home to Moldova on Friday, but he is now doubtful with a thigh injury that prevented him from training on Tuesday.
With Tottenham Hotspur's Jermain Defoe yet to start a league game this season, it means either Rooney's United team-mate Danny Welbeck or Southampton's Rickie Lambert will lead the line against Moldova.
"It is sad to see Wayne out, but it is an opportunity for someone to come in and take their chance," Walcott added. "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 and I am sure he knows what to do."