As confirmed by the club, the England winger will miss "at least six months," will require surgical repair on his knee and will need to undergo a full rehabilitation program before taking to the pitch again.
Long term, it's a serious issue for Arsene Wenger to ponder, as this sort of ailment can hinder Walcott's finest asset: blistering pace. Short term, it opens up a lot of questions, both domestically and internationally, as to who takes the winger's place in the starting XI.
Fellow professionals will never take delight in a colleague's injury, but news of Walcott's injury will have perked the ears of three players in particular.
Adam Lallana, Andros Townsend and James Milner now know they have a genuine shot at starting for England in Brazil, and it should be the former who realises this is as good a chance as he will ever get to make a certified impact on the biggest of stages.
England's 4-3-3/4-4-1-1 formation requires natural width from at least one side to bring balance to the team, and Walcott was the ideal solution.
Much like Wenger utilises a Santi Cazorla-Mesut Ozil-Theo Walcott trident—incorporating an inverted winger, a traditional No. 10 and a wide forward—Roy Hodgson relied upon Walcott for width and Danny Welbeck or Daniel Sturridge for a more central presence on the left.
That balance is absolutely key, and it's vitally important to retain it in order to sustain the flow of a side. For a perfect example, look at how Juergen Klopp fields Jakub Blaszczykowski opposite Marco Reus to widen the pitch and vary the attack.
Of the three aforementioned contenders, and incorporating Alex Oxlade-Chamberlain in the equation, Milner and "The Ox" are the most natural fits in the wide right position.
But hitting the byline (to cross and stretch the pitch) is important, and neither appears to have the pace to do so. Milner certainly doesn't, and Chamberlain wasn't the fastest even before his own ACL surgery.
What condition is he in? Not one person can make a wholesome argument one way or the other.
That leaves Townsend, who cuts inside and shoots for a living, or Lallana, who can do either and has the pace to sell dummies and fakes.
Even Southampton fans have been surprised by just how much he's improved this season after a strong 2012-13 campaign, with his sublime first touch, technique, long-range shooting and incredible agility really coming to the fore.
He's an exceptionally intelligent football player who's come on leaps and bounds under the excellent Mauricio Pochettino, and that kind of tactical nous can be useful to England in both attacking defensive phases.
Townsend is also improving his game, and only an injury layoff of his own halted some impressive progress. But of the two, Lallana feels the less raw, more ready and more impressive all-round option.
Adam Lallana will know Walcott's misfortune can be his own personal gain, and Saints fans will be hoping he turns it on consistently between now and the end of the season to force his way onto the plane and into the XI.
It's an open opportunity for four men. Who will prevail and take the chance? England fans should hope, for their own sakes and their team's, that it's the South Coast star.