A six-week absence would see Youngs miss England's first warm-up match against Wales at Twickenham on August 6 and leave him extremely doubtful for the return fixture in Cardiff a week later.
The less optimistic return date would see the 21-year-old available for selection just a couple of days before the third and final friendly against Ireland on August 27, leaving him with a little more than a fortnight to get match fit prior to England's World Cup opener against Argentina on September 10.
Youngs will require crutches for the initial stages of his recovery but the England management are convinced he will be able to play a full part a World Cup campaign that also pits the RBS 6 Nations champions against Scotland, Romania and Georgia.
And, despite the setback, there seems little danger of Youngs being overlooked for a place in Martin Johnson's 30-man squad if he proves his fitness.
While Johnson has also named Danny Care, Joe Simpson and Richard Wrigglesworth in his initial 45-man training squad, Youngs has established himself as an England favourite since his first international start in the second-Test win over Australia in Sydney a year ago.
But with his England team-mates in the early stages of a 10-week programme ahead of the tournament, Youngs will arrive in New Zealand without the same amount of preparation as the rest of the squad as he enters what will be his first senior worldwide competition.