England face Moldova and Ukraine in two crucial qualifiers over the next week, but Rooney will not feature in either fixture after a nasty collision with Phil Jones in training.
The England striker needed stitches to patch up the wound, which is roughly the length of a 500ml water bottle, according to Walcott.
On the basis on Walcott's information, Rooney, 27, might want to keep out of the public eye in the short term.
Pointing to a bottle on the top table of his press conference at St George's Park on Tuesday, Walcott 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."
Jones raised boot could not have connected with Rooney's head at a worse time for England manager Roy Hodgson.
With Rooney's club future now resolved, Hodgson was looking to the striker to provide the ammunition to shoot down Moldova and Ukraine.
After scoring five goals in Liverpool's first four matches of the season, Daniel Sturridge was the obvious replacement for Rooney, but he is unlikely to be fit for Friday's clash against Moldova because of a thigh injury.
The Liverpool striker carried the injury in to Sunday's win over Manchester United, and as a result he missed England training on Tuesday.
Luckily for Hodgson, Danny Welbeck and Rickie Lambert have started well this season, although the only other striker in the squad - Jermain Defoe - is yet to start a league game.
Walcott is sure England will have enough firepower to pick up six points even if Sturridge has to pull out, however.
"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."
As a senior member of the squad with 33 caps, Walcott will shoulder some of the responsibility when it comes to scoring goals too.
When Walcott scored a stunning hat-trick against Croatia in 2008, few would have thought that he would go on to score just two more goals in the five years that followed.
The 24-year-old concedes it is a part of his international goal tally is something he would like to improve over the coming week.
"Goals have been a big thing in my game," Walcott said.
"Getting 21 last year (for Arsenal) was more than I thought I was going to get to be honest but everything was just going so well.
"Now the next step is getting goals for England as well."
With Hodgson's men currently struggling to qualify for the World Cup, the euphoria that surrounded the England camp following that night in Zagreb is severely lacking.
Hodgson's admission that he has a small pool of international class players to choose from has only dampened the expectation around the national team as they head in to the crucial upcoming qualifiers against Moldova and Ukraine.
But with the likes of Sturridge, Ross Barkley and Andros Townsend joining up with the squad in good form, Walcott thinks there is a chance of someone repeating his Zagreb heroics either against Moldova on Friday or in Ukraine four days later.
"I came in at such a young age and I got a hat-trick against Croatia - anyone could do that against Moldova and Ukraine," said Walcott, who scored for England against Scotland two weeks ago.
"It is not going to help his looks".... I don't think much can help there, Theo.- FairyNuff