Walcott had only recently returned from an abdominal injury that sidelined him for two months, and he was just beginning to hit a rich vein of form, too. Arsenal's top scorer from last season had scored five goals in five games leading up to the 2-0 win over Spurs, where he picked up the season-ending injury.
Since Walcott was ruled out, there's been plenty of speculation about who can come in and fill the void created by the winger's absence, but I wouldn't count on Arsene Wenger doing any business this month.
It's no secret that Wenger is against the January transfer window, calling it a "distraction," and it's entirely possible that he'll choose to stick with the current group of players until the summer.
In the event that the Arsenal manager does opt against spending this month, here are four players who will need to step up in order to keep the Gunners' title challenge alive.
Although Olivier Giroud and Theo Walcott are completely different types of players, a lack of Walcott means a lack of goals. After all, the 24-year-old scored 21 goals in all competitions last season, so his goals will be sorely missed as Arsenal try to end their trophy drought in the next six months.
Although Giroud has stepped up his game significantly this season with 11 goals in all competitions, he's had somewhat of a dip recently, failing to score for over a month from November 23rd to December 29th.
After a bit of rest due to a recent injury, it's time for Giroud to find his form once again. Arsenal will need him firing on all cylinders and scoring regularly if they are to keep up with the pace at the top of the table.
Although Serge Gnabry's has only played sporadically this season, his few first-team appearances have been good enough to get Arsenal fans excited about the 18-year-old.
Back in September, he opened the scoring against Swansea and was Arsenal's main attacking threat aside from Aaron Ramsey. A month later he came on as a substitute for the injured Mathieu Flamini against Crystal Palace, winning a penalty at the start of the second half after some direct running and trickery.
Gnabry's most exciting performance for Arsenal, though, came in the club's recent 2-0 win over rivals Tottenham Hotspur in the FA Cup Third Round. The fact that he started was a surprise in the first place, but Arsene Wenger's decision made all the difference as Gnabry set up Santi Cazorla for Arsenal's opening goal and put in a Man of the Match performance.
Wenger may have planned on playing Gnabry conservatively this season, but with Walcott's injury, the young German could see some more playing time. If he continues to display the same pace, technique and football intelligence that he showed against Spurs, Gnabry could end up playing a key role in Arsenal's title challenge.
The Ox is back.
After spending five months (it was only supposed to be three) on the sidelines due to an injury suffered back in August, Alex Oxlade-Chamberlain is finally fit and ready to kick-start his season.
Although Arsene Wenger has expressed, through Arsenal's official website, that Chamberlain's future lies in a Steven Gerrard-esque central midfield position, the 19-year-old's playing style is quite similar to Theo Walcott's, as pace is one of his most dangerous assets.
Although Chamberlain enjoyed an excellent debut campaign with breakout performances against Manchester United and AC Milan, he clearly suffered from "second-season syndrome" last season. With 33 appearances and just two goals in all competitions, "the Ox" struggled to break into the first team—especially with Theo Walcott in such good form.
Now that Walcott is out, there's never been a better opportunity for Chamberlain to stake a claim for his place in the first team.
Yet another Arsenal player coming back from injury, Lukas Podolski made an immediate impact on his return with a decisive goal in the Gunners' 3-1 win over West Ham on Boxing Day.
His subsequent performance—as a striker against Cardiff—reminded us that he still wasn't back to full match fitness, as he was a non-factor up front. In fact, Arsenal looked more dangerous going forward once Podolski was replaced by Nicklas Bendtner in the second half, and it was the Dane who ended up breaking the deadlock with a late goal.
Although he doesn't seem to work in a central position, the 28-year-old's return could not have come at a better time. In Podolski, Arsene Wenger has a player who is willing to shoot from outside of the box, offering a more direct method of attack when Arsenal tend to try walking the ball into the net.
He hit double figures in goals and assists last season, but he hasn't really had the chance to make an impact on Arsenal's 2013-14 season thus far. Once he's fully fit and hitting his stride, Podolski's clinical finishing and thunderous left foot will no doubt give an edge to the Gunners' title challenge.